Purpose. The study shows social differences in the context of health-related fitness and its basic components: physical activity, somatic parameters and motor fitness. The number of siblings was adopted as a social factor related to the differences. Basic procedures. 1652 boys aged 10 to 15 years from a large city environment were tested. They were divided into: (1) only children, (2) having one brother or sister, and (3) having two or more siblings. Their leisure time physical activity (LTPA) was assessed, basic somatic parameters were measured and motor tests were carried out. Main findings. LTPA frequency and duration were very similar regardless of the number of siblings. Differences in LTPA forms and LTPA partners have been revealed. Somatic parameters and motor fitness level were significantly different depending on the number of siblings. However, the analysis of motor fitness level in separate somatic categories resulted in disappearing of motor fitness differences. Conclusions. The inequality of social positions of urban boys is connected with the differences in the level of health-related fitness but its components are socially modified to a different degree.