It is not well understood whether posttransplant diabetes mellitus (PTDM) following orthotopic liver transplantation (OLTx) alters postoperative morbidity. This study was designed to evaluate this question. All adult patients who received an OLTx between July 1985 and March 1993 (n = 497) were evaluated by retrospective chart review for evidence of PTDM after OLTx. The patients identified with PTDM (n = 26) were case matched with nondiabetic OLTx recipients based on primary liver disease diagnosis, age, gender, date of first OLTx, and survival. Liver synthetic function, number and severity of rejection episodes, graft survival, total number of hospital days within the first year post-OLTx, renal function, and number and type of infection episodes were analyzed to assess differences in morbidity between the PTDM and control patients after OLTx. Of the 497 adult patients who underwent OLTx, 26 (5.2%) were identified as having PTDM within 1 month of discharge. Factors which identified individuals at higher risk for DM after OLTx included higher pre-OLTx fasting blood glucose (P = .04); lower body mass index after OLTx (P = .02); and cyclosporine rather than OKT3 induction (P = .009). Graft survival, synthetic function, and the total number of rejection episodes during the first year were not different between the two groups. The morbidity variables of total number of days in the hospital during the first 12 months, renal function, and type and number of infections were also similar between the two groups. In summary, 5.2% of adult patients developed DM within 1 month of OLTx. Pre-existing insulin resistance, postoperative stress, and immunosuppression medications all likely contribute to the development of overt hyperglycemia after OLTx. Although PTDM can be a consequence of OLTx, it does not have a significant impact on patient outcome in the first year after OLTx. Copyright © 1996 by the American Association for the Study of Liver Diseases.