Objective: Obesity is considered a risk factor for surgical site infection (SSI). We quantified impact of body mass index (BMI) on the risk of SSI for a variety of surgical procedures.Methods: We included 2012-2017 data from the Dutch national surveillance network PREZIES on a selection of frequently performed surgical procedures across different specialties. Patients were stratified into 5 categories: underweight (BMI, <18.5 kg/m2), normal weight (BMI, 18.5-25), overweight (BMI, 25-30), obese (BMI, 30-40) and morbidly obese (BMI, ≥40). Multilevel log binomial regression analyses were performed to assess the effect of BMI category on the risk of superficial, deep (including organ-space) and total SSI.Results: Of the 387,919 included patients (ranging from 2,616 for laparoscopic appendectomy to 119,834 for total hip prosthesis), 3,676 (1%) were underweight, 116,778 (30%) had normal weight, 154,339 (40%) were overweight, 104,288 (27%) had obesity, and 8,838 (2%) were morbidly obese. A trend of increasing risk of SSI when BMI increased from normal to morbidly obese was observed for almost all surgery types. The increase was most profound in surgeries with clean wounds, with relative risks for morbidly obese patients ranging up to 7.8 (95% CI, 6.0-10.2) for deep SSI in total hip prosthesis. In chest and abdominal surgeries, the impact was larger for superficial SSI than for deep SSI.Conclusions: The results of our research provide evidence for the need of preventive programs targeting SSI in overweight and obese patients, as well as for the prevention of obesity in the general population.