A case-control design was used to investigate the effects of preexisting chronic conditions on in-hospital mortality in adult trauma patients. Cases consisted of all trauma deaths (n = 3074) that occurred in 1983 in any of the 331 acute care hospitals in California. Three to four control patients (injured survivors) were matched to each case patient on the basis of injury severity, age, and individual hospital (n = 9869). The data source consisted of hospital discharge abstract data uniformly collected on all admissions to acute care hospitals in the state. Conditional logistic regression techniques were used to estimate the relative odds of dying for patients with and without one or more of 11 preexisting chronic conditions identified as potentially detrimental to outcome. The presence of cirrhosis (relative odds = 4.5), congenital coagulopathy (relative odds = 3.2), ischemic heart disease (relative odds = 1.8), chronic obstructive pulmonary disease (relative odds = 1.8), and diabetes (relative odds = 1.2) all significantly increased the risk of dying. These data provide statistical evidence to support the recommendation of the American College of Surgeons that the presence of underlying disease be considered in decisions to triage and transfer patients to trauma centers. They also underscore the importance of underlying disease in the case-mix adjustment of case-fatality rates and the identification of unexpected deaths for quality assurance review.