Retrospective cohort study. To compare short- and long-term outcomes in obese versus nonobese patients undergoing instrumented posterolateral fusion of the lumbar spine. Obesity is an important public health issue due to the negative effects on quality of life. Some studies have shown an association between obesity and higher rates of complications and unfavorable outcomes after spine surgery. We retrospectively reviewed medical records for all adult patients undergoing 1- to 3-level posterolateral fusion for degenerative spine disease between 1992 and 2012 at a single institution. Patients were divided into obese (body mass index > 30 kg/m) and nonobese cohorts to compare complications, reoperation rates, and symptom resolution at the last follow-up. A regression model was used to estimate relative risk ratios. During the study period, 732 patients underwent lumbar fusion, with 662 (90.44%) nonobese patients and 70 (9.56%) obese patients in the cohort. Obese patients had significantly higher blood loss intraoperatively (P = 0.002) and a longer average length of stay (P = 0.022). Moreover, obesity was independently associated with a significantly increased risk of developing a postoperative complication (risk ratio 2.14; 95% confidence interval, 1.10-4.16) and surgical site infection (risk ratio 3.11; 95% confidence interval, 1.48-6.52). At the last follow-up, a higher proportion of obese patients had radiculopathy (P = 0.018), motor deficits (P = 0.006), sensory deficits (P = 0.008), and bowel or bladder dysfunction (P = 0.006) than nonobese patients. In this study, obese patients undergoing lumbar fusion had higher blood loss, longer lengths of stay, higher complication rates, and worse functional outcomes at the last follow-up than nonobese patients. These findings suggest that both surgeons and patients should acknowledge the significantly increased morbidity profile of obese patients after lumbar fusion.