This study describes the effects of regular 4-year-long rehabilitation exercises based on a 45 min exercise session 3 times/w, on changes in body composition of 23 postmenopausal women (experimental group -E) following mastectomy. Anthropometric measurements and body composition (BIA method) were compared between the E women and a control (C) group of non-exercising women (N = 20) who underwent the same surgical procedure. Following four years of regular exercise, significant increase in body cell mass (BCM) [kg, %] and intracellular water (ICW) [1] was observed (P < 0.05), whereas the content of extracellular water (ECW) [%] and extracellular mass (ECM) [kg] decreased (P < 0.05). Body mass, body mass index (BMI) and fat mass (FM) [kg, %], did not change significantly. Central fat deposition measured by waist-to-hip ratio (WHR) and the waist circumference increased (P < 0.05), whereas hip circumference did not change. Compared to the C group the E group had a lower BCM, BMI, FM [kg, %], ECW [l, %] and ECM [kg] (P < 0.05), and a higher total body water (TBW) [%], fat-free mass (FFM) [%] and BCM [%] content (P < 0.05). Hence, regular exercise may have a positive effect on the abovementioned factors which are considered among breast cancer risk factors after mastectomy.