Objectives: The purpose of the study was to assess the relationship between the body mass index and sports and physical activities in an unbiased, simple, random sample of teenagers from mid-northeastern Poland. Material and Methods: Children and adolescents aged 10–19 years old were surveyed. The Internet-based survey was conducted to collect information on physical activity, leisure time, studying hours, weight, and height. Results: Obesity was found to be low in the study population. The overall prevalence of overweight was not higher than 8%. Only 0.6% of participants were underweight. Overall, 68% of them confirmed participating in sports. Boys were frequently more active than girls (> 70% boys and > 60% girls). Participants mostly played soccer, volleyball, handball, or basketball (30.5%); 7% of them practiced martial arts and 16.5% of them participated in swimming. Sports activity significantly influenced body mass index. Obesity was not related to the lack of participation for physical activity in girls but a significant relationship was observed in boys (p < 0.02). The risk of obesity or overweight for inactive teenagers increased 2-fold (p < 0.01). Conclusions: We report the overall participation of teenagers from 2 cities in mid-northeastern Poland in sports and physical activities and demonstrate that the lack of statistically significant differences in the body mass index and obesity and overweight is prevalent among boys and girls. Obesity was significantly related to less physical activity in boys, and the likelihood of obesity or overweight increased in inactive teenagers. Thus, we suggest that the importance of overweight and obesity prevention programs should be emphasized. Int J Occup Med Environ Health 2018;31(6):771–782