Surgical site infection (SSI) following spinal surgery is a frequent complication and results in higher morbidity, mortality and healthcare costs. Patients undergoing surgery for spinal deformity (scoliosis/kyphosis) have longer surgeries, involving more spinal levels and larger blood losses than typical spinal procedures. Previous research has identified risk factors for SSI in spinal surgery, but few studies have looked at adult deformity surgeries. We retrospectively performed a large case cohort analysis of all adult patients who underwent surgery for kyphosis or scoliosis, between June 1996 and December 2005, by our adult spine division in an academic institution to asses the incidence and identify risk factors for SSI. We reviewed the electronic patient records of 830 adult patients. SSI was classified as deep or superficial to the fascia. 46 (5.5%) patients were found to have a SSI with 29 patients (3.5%) having deep infections. Obesity was found to be an independent risk factor for all SSI and superficial SSI (P = 0.014 and P = 0.013). As well, a history of prior SSI was also found to be a risk factor for SSI (P = 0.041). Patient obesity and history of prior SSI lead to increased risk of infection. Since obesity was related to an increased risk of both superficial and deep SSI, counseling and treatment for obesity should be considered before elective deformity surgery.