Data from 4225 persons from the National Health and Nutrition Examination Survey (HANES) was used to determine whether obesity was associated with osteoarthritis (OA) or joint pain. Subjects were divided into four groups on the basis of sex and race. We found that obesity was associated with OA of the knee for each sex/race group (p less than 0.01). The association was strongest for women, and it was present even for subjects without evidence of knee pain on physical examination. Frame size was not significantly associated with OA of the knee. Relative weight was weakly associated with OA of the hips in white women and nonwhite men but not significantly associated with OA of the sacroiliac joint. Diabetes did not seem to be an important risk factor for OA. These results suggest that the additional mechanical stress resulting from obesity is the principal reason for the association between obesity and OA.