BACKGROUND: The purpose of this study is to assess to what extent the effect of childhood socioeconomic status on adult health could be explained by a higher prevalence of unhealthy behaviour among those with lower childhood socioeconomic status. METHODS: Data were obtained from the baseline of a prospective cohort study in the Netherlands (13 854 respondents, aged between 25 and 74). Childhood socioeconomic group was indicated by occupation of the father, and adult health was indicated by perceived general health, health complaints and mortality. Adult socioeconomic status was measured by current occupation. Behavioural factors were smoking, alcohol consumption, Body Mass Index and physical activity. Relations were analysed using logistic regression models. RESULTS: A clear association between childhood socioeconomic circumstances and adult health was shown, as well as an association between childhood socioeconomic circumstances and health-related behaviour, even after adjustment for current socioeconomic status. Physical activity shows the strongest relation with childhood socioeconomic circumstances. Behavioural factors explain the relation between childhood socioeconomic status and adult health for approximately 10%. CONCLUSIONS: Childhood socioeconomic circumstances have an independent effect on adult health and health-related behaviour: the risk of health problems and health damaging behaviour is higher in lower childhood socioeconomic groups. The independent effect of childhood circumstances on adult health operates for a small part through unhealthy behaviour.