The purpose of this study was to assess the clinical and radiographic outcomes of primary total knee arthroplasty (TKA) in morbidly obese patients compared to a cohort who had a normal body mass index (BMI). We reviewed 105 knees in 84 patients who had a minimum BMI of 40 kg/m2 who underwent a primary TKA between 2006 and 2010. There were 17 men and 67 women who had a mean age of 59 years and a mean follow-up of 52 months. Outcomes evaluated included implant survivorship, Knee Society scores, activity scores, and complications. Kaplan-Meier analysis demonstrated statistically similar overall implant survivorship between the morbidly obese and the comparison groups (96 vs 97%). However, the mean Knee Society objective (85 vs 91 points) and functional scores (84 vs 89 points), as well as activity scores (4.2 vs 6.1 points) were lower and complications were higher in morbidly obese patients at final follow-up. It is encouraging that in the morbidly obese patients, total knee arthroplasty can have acceptable results at midterm follow-up, however, these patients may benefit from preoperative counseling to have realistic expectations from their surgery.