A high correlation between continuous measures of self-reported and measured weight and height has led investigators to infer that self-report of these variables is appropriate in epidemiologic studies. We evaluated the sensitivity and specificity of categorical definitions of body mass defined using self-reported height and weight on 7,455 adult participants of the Lipid Research Clinics Family Study (1975-1978) on whom both self-reported and measured height and weight were available. The categorical definition of obesity used here was a body mass index of at least 30 kg/m2. Overall, the sensitivity of the obese category when defined with self-reported weight and height was 74% (95% CI = 72%-76%), and the specificity was 99%. The sensitivities of the categories defined using self-reported measures varied considerably by sex, age, and educational level. Overall, the sensitivities were higher for women compared with men, as men were less accurate in reporting height. The sensitivity of the categorical definitions of obesity decreased with increasing age and education in both men and women. The sensitivity for elderly obese men was below 50%. These results suggest that substantial misclassification can occur when self-reported information is used to define body mass categories.