The aim of this study was to evaluate midparental BMI among intergenerational factors associated with obesity in adult offspring. The data was from an unusual two-generational observational design of 1,477 married couples from Renfrew and Paisley in Scotland who were aged 45-64 years when screened in 1972-1976, and 1,040 sons and 1,298 daughters aged 30-59 years when screened in 1996. BMI was categorized as normal (< 25 kg/m(2)), overweight (25-29.9 kg/m(2)), and obese (> or = 30 kg/m(2)) in offspring and parents. Midparental BMI was defined as the mean of the mother's and father's BMI. Low physical activity, nonsmoking status, higher cholesterol level, and manual social class were all associated with increased BMI in offspring. The effect of reported dietary intake was less clear. Offspring of obese parents (defined by midparental BMI) were over four times more likely to be obese than offspring of normal weight parents. Midparental BMI had a strong effect on offspring BMI, independent of social class, smoking habit, physical activity, and reported dietary intake. Adding midparental BMI to the regression model more than doubled the explained variation of offspring BMI from 7.7 to 17%. Every 1 kg/m(2) increment in midparental BMI was associated with a BMI greater by 0.51 kg/m(2) in offspring. We conclude that midparental BMI is a useful simple tool to predict offspring BMI. Whether it represents genetic or environmental family effects, it is easily ascertained by the individual and could be used in health promotion and clinical settings to target individuals who are at increased risk of becoming obese.