The epidemic of obesity, which is one of the most important public health problems, appeared paradoxically as a result of improving living conditions. The aim of this study was to determine the effects on overweight/obesity prevalence of the primary-school-based intervention program. The group of 5,293 children (7-11 year-olds) from Poznań (Poland) was divided into two subgroups: experimental and control one. The research group was participating in extra physical activities. Measurements (height and weight) were taken twice: at baseline and after 1-year follow-up. The estimations of the prevalence of overweight/obesity were based on the cut-off points of the IOTF values. To estimate the risk the odds ratio (OR) were calculated. There were no differences in BMI for both boys and girls. Also there were no significant differences in prevalence of overweight and obesity, for both sexes. The risk of being overweight/obese was not reduced in children in the experimental group - OR for boys was 0.93 (0.80, 1.08), and for girls OR = 0.88 (0.76, 1.03). In conclusion, the risk of overweight/obesity has not changed after one year of extra physical activities and engagement in health-oriented education program. This study shows that in case of such programs it is necessary to apply more intense interventions, probably also during longer period of time. It is possible that other adverse factors have a stronger influence on the body mass, which would suggest that the theoretical part of intervention concerning pro-health-related behaviors was not implemented in practice.