Objective: To define risk factors that predict adverse outcomes after the surgical treatment of appendicitis in Department of Veterans Affairs Medical Centers. Summary Background Data: Risk factors for adverse outcomes after the surgical treatment of appendicitis in adults are poorly defined. Accurate presurgical assessment of the risk of perioperative complications and death is important in planning surgical therapy. Methods: The VA National Surgical Quality Improvement Program contains prospectively collected and extensively validated data on ∼1,000,000 major surgical operations. All patients undergoing surgical intervention for appendicitis from 1991 to 1999 registered in this database were selected for study. Independent variables examined included 68 putative preoperative risk factors and 12 intraoperative process measures. Dependent variables were 21 specific adverse outcomes, including death. Stepwise logistic regression analysis was used to construct models predicting 30-day morbidity rate and the 30-day postoperative mortality rate. Results: There were 4163 patients identified. The mean age was 50 years; 96% were male. Sixteen percent of patients had 1 or more complications after surgical intervention. Prolonged ileus, failure to wean from the ventilator, pneumonia, and both superficial and deep wound infection were the most frequently reported complications, accounting for the majority of the morbidity. The 30-day mortality rate was 1.8% (74 deaths). For >50% of the complications reported, the 30-day mortality rates were significantly higher (P < 0.01) for patients with complications than for those without. Thirty-day mortality rates for several complications exceeded 30%, including cardiac arrest, coma >24 hours, myocardial infarction, acute renal failure, bleeding requiring >4 units of red cells, and systemic sepsis. Four preoperative factors predicted a high risk of 30-day mortality in the logistic regression analysis: "completely dependent" functional status, bleeding disorder, steroid usage, and current pneumonia. "Threat to life" or "moribund" American Society of Anesthesiologists classification and more than a 10% weight loss in the 6 months before surgery were associated with a high risk of complications. Conclusions: Morbidity and mortality rates after the surgical treatment of appendicitis in VA hospitals are comparable with those reported in other large series. Most postsurgical complications are associated with an increased 30-day mortality rate. The models presented here are the most robust available in predicting 30-day morbidity and mortality for VA patients with appendicitis. Furthermore, they provide a starting point for the design of similar models to evaluate non-VA patients with appendicitis using the data the National Surgical Quality Improvement Program is currently gathering from private hospitals.