To study the relationship between patient weight and perioperative morbidity, 512 total knee arthroplasties performed in 406 patients were reviewed. Patient height, weight, medical history, length of hospital stay, discharge destination (home versus rehabilitation facility), and all complications were recorded. Height and weight were used to calculate a body mass index (BMI) for each patient. Examination of patient data ordered by BMI established positive correlations between BMI and a cardiac history (P=.02), a history of diabetes mellitus (P=.006), postoperative hospital stays >7 days (P=.03), discharge to a rehabilitation facility (P=.02), and the risk of a postoperative complication (P=.004). Further statistical examination revealed the greatest differences in patient data exist between patients with a BMI >35 and patients with a BMI < or = 35. Patients with the greater BMI (>35) had significantly higher rates of cardiac conditions (56% versus 33%, P=.0001) and diabetes mellitus (10.5% versus 4.1%, P=.03) than patients with a lower BMI (< or = 35). Although there were no significant differences in the rates of specific complications between the two groups, patients in the heavier group were more likely to experience a complication (38% versus 25%, P=.002) and multiple complications (9.3% versus 6.2%, P=.03) than patients in the lighter group.