Hospital discharge data for all adult trauma admissions (n = 27,029) to acute care hospitals in a five-county area were used to examine the effect of pre-existing chronic conditions (PECs) on length of hospital stay (LOS). The per cent increase in mean LOS and the relative odds of an extended LOS (i.e., greater than or equal to 3 weeks) were estimated for surviving trauma patients with and without one or more of 11 pre-existing medical conditions, taking into account both the ISS severity of injuries and patient age. Nineteen per cent (n = 5,224) of the trauma cases had one or more of the 11 PECs; 5% (n = 1,384) had two or more PECs. Those with one or more PECs stayed significantly longer (p less than 0.001) than those who had none. Mean LOS was shown to be 69% higher for those with versus those without PECs. Furthermore, patients with PECs were more than twice as likely as those without PECs to have stayed in the hospital 3 weeks or longer. The size of the effect, however, varied by patient age and injury severity; the effect of PECs on length of stay was larger among younger patients and patients with less severe injuries. With regionalization, trauma centers treat the majority of these patients and potentially suffer disproportionate financial losses when reimbursement is based on DRGs.