Estimating whether adolescents' sedentary behaviour and their lack of physical activity is determined by family characteristics or socio-cultural aspects and their impact on health and adiposity level. 932 adolescents were surveyed. Information regarding physical activity, sedentary behaviour, anthropometric index and family characteristics (structure, dynamics and functioning) was estimated by adolescents answering a questionnaire. The sample's average age was 16.07±1.09 years old, 56.3 % were female, 66.3 % had a low level of physical activity and 51.9 % were sedentary. A lack of physical activity occurred more frequently in females (56.8 % cf 41.5 %: p=0.000); family characteristics did not influence such risk behaviour. A relationship between physical activity and sedentary lifestyle with BMI could not be demonstrated, whilst waist circumference was associated with risky behaviour patterns. Teenagers in good health were more active (36.1 % cf 27 %) and less sedentary (49.3 % cf 59.4 %) than those for whom an unhealthy state was reported. Sedentary behaviour and a lack of physical activity were more determined by socio-demographic factors than family aspects, such behaviour pattern having a direct influence on the adolescents' health.