A retrospective study of patients who had had a primary total hip arthroplasty without cement between 1983 and 1990 was done to examine differences in clinical and radiographic results between obese and normal-weight individuals. Patients were considered to be of normal weight if the body-mass index (calculated as weight in kilograms divided by height in meters squared) was between twenty and less than thirty, and they were considered to be obese if the body-mass index was thirty or more. There were 127 normal-weight patients (142 hips) and fifty-five obese patients (sixty hips) in the analysis. Eight obese patients (eight hips) who had a body-mass index of forty or more were considered to be morbidly obese and were examined separately as a subset of the obese group. The duration of follow-up averaged forty-eight months (range, twenty-four to ninety-two months). There were no differences between the groups with regard to age, diagnosis, the type of stem or cup, the type of bone, the postoperative level of activity, or the duration of follow-up. The obese patients had a significantly greater loss of blood during the operation than the patients in the normal-weight group. There were no significant differences between groups with regard to the prevalence of perioperative complications, the number of units of blood transfused, the operative duration, or the duration of hospitalization. Of the 202 hips, 184 (91 percent) were pain-free or only mildly painful at the latest follow-up examination.(ABSTRACT TRUNCATED AT 250 WORDS)