Health care reform will affect the relationship of trauma centers to health maintenance organizations and other managed care plans. We studied Kaiser Permanente Medical Center (Kaiser) members admitted to the Trauma Center at San Francisco General Hospital (SFGH) to determine: (1) variables predicting transfer from SFGH to a Kaiser Hospital (repatriation), (2) the length of hospital stay (LOS), and (3) the cost of their care. The SFGH trauma registry provided data on 7,794 patients admitted before 1994. To investigate LOS, 89 Kaiser patients over 1 year were matched with non-Kaiser patients on age, maximum Abbreviated Injury Scale score (MAIS) by body region, Injury Severity Score (ISS), head injury severity, and blunt or penetrating injury and disposition. Kaiser patients were significantly younger, more likely to have blunt injury, and had a lower death rate. Significant predictors of repatriation were an MAIS score > or = 3, abdominal or extremity injury, and an ISS score of 26 to 40. The mean LOS for all Kaiser patients was 7.6 days, compared with 4.8 for controls (p = 0.20). However, mean LOS was significantly longer in repatriated Kaiser patients compared with controls (16 vs. 7.8 days, p < 0.0005). Kaiser reimbursement rates were comparable with commercial payors, but higher than others. A relatively small number of severely injured patients account for a large percentage of costly trauma care. Analyses of patient subsets are necessary for trauma centers to negotiate suitable relationships with managed care plans. A prospective study is needed to examine the cost efficiency of early transfer of managed care patients.