Dietary patterns have been identified in adults, but less is known about children and adolescents. For the first time, we have investigated lifestyle patterns combining diet and physical activity in 12-year-old French preadolescents and examined their association with sociodemographic factors. Physical activity, sedentary activities and dietary habits were assessed by questionnaires given to 2724 students in 2001. Family income tax and parents' educational level, as indicators of socio-economic status, and the size of the residence commune were obtained from parents. After adjusting for socio-economic status, physical activity was positively associated with a consumption of frui/egetable/ruit juice on more than four occasions in the previous 24h (P<0·001). Sedentary activities were positively associated with the consumption of French fries or potato chips (P<0·001), with sweetened drink as the most usual drink (P<0·001) and with nibbling while watching television (P<0·001), and inversely associated with a high consumption of frui/egetable/ruit juice (P=0·04). Multiple correspondence analysis identified two independent axes and specific combinations of behaviour: one axis characterised by sedentary activity, sweetened drink as the most usual drink, the consumption of French fries or potato chips and nibbling while watching television; a second one associating physical activity and the consumption of frui/egetable/ruit juice. Both socio-economic proxies were associated with the former axis (P<0·001). The size of the residence commune was associated with the latter (P<0·1). Combinations of diet and physical activity habits were identified in adolescents, indicating that prevention programmes targeting both behaviours may have an enhanced outcome.