We sought to examine the relationship of body mass index (BMI) at age 18 years with the degree and rate of rise in body weight during adulthood among the morbidly obese. We evaluated 196 patients with a standard medical history form and a structured interview with questions regarding weight at age 18 years. The study included 40 (20.4%) men and 156 (79.6%) women. The mean BMI was 50.2+/-8.0 kg/m2, range 37.0-80.0 kg/m2. Based on self-reported weight, 133 (67.9%) were overweight/obese (BMI >25 kg/m2) and 68 (34.7%) were obese (BMI > or =30 kg/m2) at age 18 years. The distribution of cumulative weight gain was normal with a mean of 60.8+/-23.7 kg. There was a positive relationship (r=0.36, p<0.0001) between BMI at age 18 years and BMI in adulthood at a mean of 44+/-10.6 years. Independent predictors for cumulative adult weight gain were BMI at age 18 years (p<0.0001); women (p<0.0001); African Americans (p=0.05). These data suggest that modestly overweight young adults can have excessive weight gains during adult life, resulting in morbid obesity and high rates of obesity-related comorbidities.