Several groups have reported that recipients of a simultaneous pancreas-kidney transplantation suffer from more kidney rejection episodes than do recipients of a kidney transplantation (1-6). However, it is not known whether this is interstitial rejection, vascular rejection, or both. In this study, the renal biopsies and transplantectomies of 45 pancreas-kidney and 48 kidney transplant recipients were evaluated for the presence of interstitial and vascular rejection. Furthermore, the influence of OKT3 induction therapy on rejection after pancreas-kidney transplantation was studied. Of the 45 pancreas-kidney recipients. 4 patients did not suffer from rejection during follow-up, 28 suffered only from interstitial rejection, and 13 suffered from vascular (with or without interstitial) rejection, whereas 12, 19, and 14 of the 48 kidney transplant patients had no rejection, interstitial rejection, or vascular (with or without interstitial) rejection, respectively. Three patients with a kidney transplant were treated for rejection although no biopsy was taken. In the pancreas-kidney group, 38 of the total of 149 biopsies and transplantectomies taken contained no rejection, 92 had interstitial rejection, and 19 had vascular rejection. In the kidney group, these values were 13, 41, and 25, respectively, of 79 biopsies and transplantectomies taken (P = 0.002). Five-year renal graft survival was 79% in the kidney group and 60% in the pancreas-kidney group. Renal graft survival rates differed significantly (P = 0.02). Renal graft survival and occurrence of rejection did not reach significance between pancreas-kidney recipients treated with OKT3 induction therapy and pancreas-kidney recipients receiving conventional triple therapy.(ABSTRACT TRUNCATED AT 250 WORDS)