To explore the relationship between several physical activity (PA) and dietary behaviours and BMI Z-score and to investigate the relationship between changes in these variables and in the BMI Z-score over a 4-year period from childhood to adolescence.

Longitudinal study in which children were included in the fifth grade and measured for four consecutive years. Dietary and PA behaviours as well as height and weight were measured by means of self-reported validated questionnaires.

Fifty-nine Flemish elementary schools.

The baseline sample consisted of 51·9 % boys and the mean age was 10 (sd 0·4) years. During the first measurement year (2002), data on 1670 fifth graders were gathered. These measurements were repeated after 1 (n 1557), 2 (n 1151) and 3 (n 807) years.

Significant inverse relationships with BMI Z-score were observed for frequency of breakfast consumption (β = −0·033, se = 0·012) and frequency of sports participation (β = −0·011, se = 0·004) across four time points. Significant inverse relationships between changes in BMI Z-score and changes in frequency of sports participation (β = −0·011, se = 0·006) and hours of physical education (PE; β = −0·052, se = 0·023) were observed, meaning that decreases in sports participation and hours spent in PE were associated with increases in BMI Z-score.

The present study provides an important insight into different dietary and PA behaviours related to (changes in) BMI Z-score during the transition from childhood to adolescence.