The aim of this study was to assess whether a school-based program with expanded physical education lessons was effective in increasing children's physical capacity and in preventing excessive weight gain in children. The study performed in 2000-2003 comprised 132 children, 73 boys and 59 girls at baseline 6-9 years and in follow-up 9-12 years, attending two different schools with a similar size, appearance and structure in a rural area. The norm school (N-school) followed the stipulated curricular time, one to two physical education lessons a week, while the intervention school (I-school) increased it to four lessons. More positive changes in physical index (the sum of the age-standardized results in 11 physical tests) were found among children in the I-school than in the N-school. The number of children who increased body mass index (BMI) increased in both schools, but a lower increase in BMI could be seen in the I-school. Expanded physical education lessons could increase physical status among both overweight and normal-weight children, in particular aerobic fitness. The weekly dose of physical activity must be higher than 40 min a day and must start earlier in children's life to be more effective in combating BMI increase.