Background: Systematic physical activity can permanently prevent disadvantageous developments in the human body. This is very important especially for women, for whom the maintenance of a lean body in good shape is sometimes a primary consideration. However, in most cases, this activity is taken randomly and does not produce the desired effects such as reducing body fat. The purpose of the study was to evaluate changes in female body composition induced by 12 weeks of swimming training compared to sedentary controls. Methods: Training sessions occurred three times per week (60 min/session). Height, body mass, and waist/hip circumference and waist/hips ratio (WHR) were measured. Body cell mass (BCM), total body water (TBW), extracellular (ECW) and intracellular water (ICW), fat mass (FM), lean mass (FFM), and muscle mass (MM) were measured using bioelectrical impedance (pre/post). Results: Training elicited decreases in hip circumference and increase in WHR. No changes were recorded in BCM, TBW, ECW, ICW, FM, FFM, and MM. Controls experienced decreases in values of BCM, ICW, and MM and increases in ECW. Conclusion: The applied swimming training did not significantly affect the body composition parameters. Inactivity also triggered a tendency toward unhealthy movement of water from the intracellular to extracellular space. The purpose of the study was to evaluate changes in female body composition induced by 12 weeks of swimming training compared to sedentary controls. Methods: Training sessions occurred three times per week (60 min/session). Height, body mass, and waist/hip circumference and waist/hips ratio (WHR) were measured. Body cell mass (BCM), total body water (TBW), extracellular (ECW) and intracellular water (ICW), fat mass (FM), lean mass (FFM), and muscle mass (MM) were measured using bioelectrical impedance (pre/post). Results: Training elicited decreases in hip circumference and increase in WHR. No changes were recorded in BCM, TBW, ECW, ICW, FM, FFM, and MM. Controls experienced decreases in values of BCM, ICW, and MM and increases in ECW. Conclusion: The applied swimming training did not significantly affect the body composition parameters. Inactivity also triggered a tendency toward unhealthy movement of water from the intracellular to extracellular space. swimming training body composition aerobic exercise pmc-status-qastatus 0 pmc-status-live yes pmc-status-embargo no pmc-status-released yes pmc-prop-open-access yes pmc-prop-olf no pmc-prop-manuscript no pmc-prop-legally-suppressed no pmc-prop-has-pdf yes pmc-prop-has-supplement no pmc-prop-pdf-only no pmc-prop-suppress-copyright no pmc-prop-is-real-version no pmc-prop-is-scanned-article no pmc-prop-preprint no The study of body composition in conjunction with the analysis of endocrine have become an important element not only assess the risk of diseases associated with obesity, but also in their prevention and treatment (e.g., diet, exercise/training) [ 3 ]. In competitive sport the knowledge of body tissue proportions is essential to determine the morphological characteristics of an athlete. The opportunity for excellence in sport practice depends on this very information [ 3 , 4 , 5 ]. Low physical activity of humans leads to changes in the proportions of the basic components of the body [ 6 , 7 ]. It is well known that this refers mainly to the increase in body fat [ 8 ]. Numerous studies have shown that excess body fat and its specific depositing, depending on gender, significantly increases the risk of ischemic heart disease, diabetes, hypertension, or various forms of cancer [ 9 , 10 , 11 , 12 , 13 , 14 ]. The effects of an imbalance in the body of individual body components affects people of all ages. Preservation of the content of body components from an early age ensures the maintenance of good health later in life, when the metabolic processes are slowed down [ 15 , 16 , 17 ]. Moderate, but systematic physical activity can permanently prevent disadvantageous developments in the human body [ 18 , 19 ]. where 20 trained girls (16.1 ± 1 year, BMI = 25.1 ± 1.8 kg/m 2 ) took part in 10-week swimming training and were divided into two groups traditional periodization (BP) and reverse periodization (RP) [ 36 ]. The authors recorded a significant increase in FFM ( p < 0.05) and decrease in FM ( p < 0.05) within BP group while they did not notice any changes within RP group [ 36 ]. Siqueira et al. did not observe any changes in body composition of women ( n = 133) with rheumatoid arthritis participating in a 16-week controlled program consisting of different kind of aquatic exercise (3 sessions per week) [ 37 ]. Although, Penaforte et al. In the case of the women participating in this experiment we found TBW values lower than the norm (normal: 62%) (participant values: 55.5 ± 2.9%). It was noted that the 12 weeks of swimming training elicited a slight decrease in TBW for all the exercising women, with a concurrent slight increase of its percentage (average: 0.18 ± 1.6%). Similar results were obtained by Quiterio et al. [ 44 ] for young female athletes (13 years) undergoing training of varying duration (<4.5h/week, 4.5–8.9 h/week and 9 h/week). The 12-week swimming training caused a negligible decrease of both ECW and ICW. It is undeniable that for exercise to bring expected results, it should be supported by an appropriately selected diet, especially bearing in mind age and the health condition. Control of the effectiveness of training and its impact on the body composition of the participants of such training in conjunction with a healthy diet will be the subject of our research in the future. 6. Conclusions Prolonged aerobic training (12 weeks of swimming) caused no significant change (decrease) in body weight of the tested women. The applied training resulted in an insignificant decrease in body fat tissue, with a concurrent an insignificant increase in lean body mass, (including muscle mass).