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To identify preoperative factors associated with an increased risk of postoperative pneumonia and subsequently develop and validate a risk calculator.The American College of Surgeons' National Surgical Quality Improvement Program, a multicenter, prospective data set (2007–2008) was used. Univariate and multivariate logistic regression analyses were performed. The 2007 data set (N=211,410) served as the training set, and the 2008 data set (N=257,385) served as the validation set.In the training set, 3825 patients (1.8%) experienced postoperative pneumonia. Patients who experienced postoperative pneumonia had a significantly higher 30-day mortality (17.0% vs 1.5%; P<.001). On multivariate logistic regression analysis, 7 preoperative predictors of postoperative pneumonia were identified: age, American Society of Anesthesiologists class, chronic obstructive pulmonary disease, dependent functional status, preoperative sepsis, smoking before operation, and type of operation. The risk model based on the training data set was subsequently validated on the validation data set, with model performance being very similar (C statistic: 0.860 and 0.855, respectively). The high C statistic indicates excellent predictive performance. The risk model was used to develop an interactive risk calculator.Preoperative variables associated with an increased risk of postoperative pneumonia include age, American Society of Anesthesiologists class, chronic obstructive pulmonary disease, dependent functional status, preoperative sepsis, smoking before operation, and type of operation. The validated risk calculator provides a risk estimate for postoperative pneumonia and is anticipated to aid in surgical decision making and informed patient consent.