Background: Motivation is what drives all aspects of human functioning - from the physiological needs to self-realization. It has, therefore, great impact upon undertaking physical activity and may be connected with various factors such as attitudes and role models taken from home, but also with social influences later in life e.g. peers’ opinion. Material/Methods: 400 students of the lower-secondary schoolsin GdyniaandGdańsktook part in the research. Each of them received a questionnaire with questions regarding their and their parents’ / ’guardians’ physical activity. In statistical analysis Cross Tabulation (χ2 statistics) and Frequency Tabulation were used. Results: Undertaking physical activity is more strongly related with the male sex. The factors which had the strongest influence on undertaking sport behaviours in teenagers turned out to be: influence of a peer group and satisfaction with undertaking these behaviours. Among the weakest ones were: siblings, offer of sport clubs as well as Physical Education teachers. It was also proved that the father’s influence on undertaking physical activity by children is greater than the mother’s. Conclusions: The main factors which influence interest in sport in adolescents are consecutively the influence of peers and satisfaction from the sport itself.