OBJECTIVE: To investigate associations between body mass index (BMI) and family characteristics, including lifestyle, in parents and offspring from Australian families.DESIGN AND SUBJECTS: Longitudinal survey of 219 families of Australian children who had been surveyed 3-yearly between the ages of 9 and 18y.MEASUREMENTS: Socio-economic status, weight and height, diet from 3 day records or food frequency questionnaires, alcohol consumption, smoking habits and physical fitness in offspring (bicycle ergometry in 18-y-olds).RESULTS: In 18-y-olds, in models examining offspring's lifestyle variables, BMI was predicted negatively by physical fitness (P = 0.012), and positively by alcohol intake (P = 0.046) in sons while, in daughters, only a negative association with physical fitness was significant. In models including parental characteristics, BMI in 18-y-old sons and daughters was significantly predicted by mothers' and fathers' BMI, independently of offsprings' alcohol intake, smoking, physical fitness and parents' education, and, in daughters, by fathers' alcohol intake. These models explained 48% of variance in daughters and 33% in sons. In both sons and daughters, BMI over the 9y of the survey was consistently higher in offspring with overweight or obese fathers (P = 0.033 for sons, P = 0.024 for daughters) or mothers (P = 0.031 for sons, P = 0.037 for daughters). Physical fitness at the ages of 12, 15 and 18y was negatively related to fathers' obesity in daughters and mothers' obesity in sons. Obesity in fathers was associated with a four-fold increase in risk of obesity at the age of 18y in both sons and daughters with an independent eightfold increase in risk for daughters if mothers were obese. Birthweight was unrelated to overweight or obesity in the 18-y-olds. Alcohol intake in sons related significantly to alcohol intake in either parent while, for daughters, there was a significant association only with fathers' alcohol consumption. In daughters, fat intake was positively associated with fat intake score in both fathers and mothers.CONCLUSION: Parental overweight or obesity may identify children at risk for a range of unhealthy behaviours. Promotion of a healthy lifestyle targeting overweight families, particularly in lower socio-economic groups, should be a priority.