The association between obesity and perinatal, constitutional and social factors was studied in 1363 children (676 males, 687 females) living in six areas of north-east Italy. The children were randomly selected from four age categories (4,8,10 and 12 years of age). After adjustment for age, significant associations between the risk of obesity in the child and their body weight at birth (P < 0.01) and the father's or mother's body mass index (P < 0.001) were found in both genders. When these parental and perinatal variables were included as independent variables in a multiple logistic regression model controlling for the effect of age, parental body mass index and children's birth-weight remained independently associated with childhood obesity. In females, an interaction between birthweight and the mother's body mass index on the prevalence of obesity in childhood was found. In conclusion, parental obesity and birthweight represent major risk factors for obesity among children in north-east Italy.