Aims of the studies presented in this thesis were to assess: a) physical fitness of athletes with mental retardation (MR) comparing with individuals included in recreational and leisure activity programs (non- athletic people); b) contribution of sport specialization (basketball vs track and field) on athletes' fitness; c) contribution of selected factors to athletics and basketball performance; d) basketball abilities before and after a training period during one and two following sports seasons; and e) correlation of each fitness variable and sport abilities with subjects' MR levels. All our investigations showed that physical activity and sport training could improve fitness of individuals with MR, decreasing health risks. Moreover, the possibility to determine the contribution of selected factors to sport performance should be addressed in training to help athletes to perform successfully in their competitions.