We studied sociodemographic and behavioural factors as predictors of weight gain in 12,669 adult Finns examined twice with a median interval of 5.7 years. The association of these factors with the prevalence of obesity (body mass index greater than or equal to 30 kg/m2) was also studied in a subsequent cross-sectional survey of 5673 Finns. In uni- and multivariate analyses, the risk of substantial weight gain (greater than or equal to 5 kg/5 years) was greatest for persons with a low level of education, chronic diseases, little physical activity at leisure or heavy alcohol consumption, and for those who got married or quit smoking between the examinations. Parity and energy intake predicted weight gain in women. The prevalence of obesity was inversely associated with the level of education and physical activity, and positively associated with alcohol consumption in men and parity in women. There were no significant differences in the prevalence of obesity by smoking or marital status. The recognition of socioeconomic and behavioural factors as important determinants of weight gain and overweight helps the planning of effective treatment and preventive programmes tailored for subjects at highest risk of obesity.