We examined the association of degree and duration overweight, dietary habits and exercise with non-insulin dependent diabetes mellitus risk in a defined population of 886 men and 1114 women who were aged 50 years and older when examined in 1984-1987. After an oral glucose tolerance test, 142 men and 142 women were classified as diabetic using WHO criteria. Compared to those with appropriate childhood weight, reported underweight as a child significantly increased the rate of diabetes as an adult (RR = 1.3, P less than 0.05). Underweight as a teenager was also associated with an increased rate (RR = 1.3, P less than 0.05). Underweight as a teenager was also associated with an increased rate (RR = 1.4, P less than 0.01). In adults with current body mass indices (weight/height2) greater than 26, the diabetes rate was significantly higher for those underweight as children (RR = 1.7, P less than 0.01). A multivariate logistic regression analysis of adult diet and weight behaviors, adjusting for age and current smoking, found that a weight gain or fluctuation between the ages of 40 and 60 of 10 lbs or more significantly increased the diabetes rate (RR = 1.4, P less than 0.05; RR = 1.7, P less than 0.01). Weight gain between age 18 and the 1984-1987 visit also significantly increased the rate (RR = 1.4 per 17.3 percent, P less than 0.001). Exercise as the only means to control weight was associated with a significantly reduced diabetes rate (RR = 0.05, P less than 0.05).