Five hundred fifty patients who underwent primary total knee arthroplasty between 1987 and 2004 with a primary diagnosis of osteoarthritis and 1-year outcome data (Western Ontario and McMaster Osteoarthritis Index [WOMAC]) were evaluated. Patients were stratified into body mass index categories based on the World Health Organization classification of obesity. Patients were dichotomized into a class III morbidly obese group and a non-morbidly obese group. Independent t test and multivariable linear regression were used to determine if a difference existed in the 1-year WOMAC outcome between morbidly obese patients and all other patients. Although 1-year outcomes were worse for morbidly obese patients (P < .05), they showed greater improvement in function compared with non-morbidly obese patients. Morbid obesity does not affect 1-year outcomes in patients who have had a total knee arthroplasty.