There is a paucity of data describing the incidence of pre-existing diseases or risk factors and their effects in trauma patients. We conducted a prospective study to determine the incidence of such factors in critically ill trauma patients and to evaluate their impact on outcome. The study, performed over a 2-year period, examined the hospital course of all trauma patients admitted to the ICU. Multiple risk factors were evaluated and analyzed via multivariate regression analysis. Outcome was evaluated by infection rate, hospital length of stay, ventilator days, and mortality matched for age and Injury Severity Score (ISS). A total of 1172 patients (73% blunt injury) were enrolled over the study period. Of these, 873 (74.5%) were male. The mean age was 42.5 years with an ISS of 19.8. Tobacco use (24%) was the most common risk factor identified, followed by hypertension (HTN, 17%), coronary artery disease (9%), chronic obstructive pulmonary disease (COPD)/reactive airway disease (4%), non-insulin-dependent diabetes (NIDDM) (4%), insulin-dependent diabetes (IDDM) (3.2%), cancer (3%), liver disease (2%), and HIV/AIDS (1.4%). Of these risk factors, IDDM was found to be an independent risk factor for infection (0.004) and ventilator days (0.047), increasing age was found to be an independent risk factor for hospital length of stay (0.023) and mortality (<0.001), and HTN was found to be an independent risk factor for increased ventilator days (0.04). In addition, COPD/reactive airway disease was found to be an independent predictor of ventilator days, infection, and ICU days (P < 0.05). Thus, increased age, IDDM, COPD, and HTN are most predictive of outcome in critically ill trauma patients. With our aging population it is becoming increasingly important to identify pre-existing risk factors on admission in order to minimize their effects on outcome.