The 10-year results of primary total knee arthroplasty in patients who were obese were evaluated. In a 2-year period, 120 patients had 165 primary, posterior stabilized, cemented total knee replacements with metal backed tibial components. After exclusions, 56 patients (73 knees) were available for study. Twenty-two patients (32 knees) were classified as obese, and 34 patients (41 knees) were classified as nonobese. At 10 years followup, patients who were obese had lower Knee Society function scores than had the patients who were nonobese (mostly attributable to more difficulty with stair climbing). Patients who were obese had poorer patellar scores at 10 years. Bilaterality was more common in patients who were obese and preoperative knee scores were lower. Minor nonprogressive radiolucent lines were seen more commonly in the patients who were obese. Overall, however, 10-year Hospital for Special Surgery scores and Knee Society scores for patients who were obese were comparable with scores for patients who were nonobese and given lower preoperative Hospital for Special Surgery scores, improvement in knee score was greater. Revision rates in patients who were obese were not higher than in patients who were nonobese at 10 years followup. More study is needed of the subgroup of patients who were morbidly obese.