OBJECTIVE: To find predictors of abdominal obesity (defined by >90th percentile of waist/hip ratio (WHR)) and related factors among 31-y-old men and women. DESIGN: Longitudinal study of the northern Finland birth cohort of 1966 with measurements obtained at birth, 14 and 31 y. SUBJECTS: A total of 2841 men and 2930 women with data on WHR at 31 y. RESULTS: The most important predictor of abdominal obesity among the 31-y-old men was a high body mass index (BMI), those with normal weight at 14 y who were obese at 31 y having an especially high risk of abdominal obesity at 31 y. Abdominal obesity was independently associated with current weight status, small size for gestational age, a high intake of alcohol at 31 y, physical inactivity at 31 y, unhealthy diet in the sense of infrequent consumption of fiber-rich foods and frequent consumption of sausages, and a low level of occupational training. Physical inactivity and minimal vocational training also tended to be associated with abdominal obesity among women. The analyses were controlled for maternal age and BMI, and also for hormonal contraception and parity among women. CONCLUSIONS: Some aspects of risk of adult abdominal obesity were evident during adolescence, and good advice is needed then, and in early adulthood, in order to reduce the risk of abdominal obesity in their thirties. Those who are small for gestational age are vulnerable to the development of abdominal obesity. Successful weight control from adolescence to adulthood, and healthy eating, alcohol drinking and exercise habits are important for avoiding abdominal accumulation of body fat.