An organized intervention program in pre-pubertal children based on increased physical education and activity together with nutritional advice to the children and their families may prevent obesity. Children aged 9 to 11 received double hours of weekly physical education lessons and additional nutritional advice for 2 years. They were compared with a similar group who received standard physical education lessons only. A total of 82 study and 26 control children completed the 2 years of the project. Boys in the control group gained significantly more weight than the corresponding study group (8.4 kg vs 3.2 kg, respectively (P<0.016)), control delta body mass index (BMI) 1.02 vs study 0.44 (P<0.012). There was no significant change in the weight or BMI in the girls. We conclude that the prevention of obesity may be attained by similar programs. Hormonal effects and compliance at this age may be responsible for the sex differences in this study.