Over the past few years, we have observed a substantial increase in the number of patients followed at our hospital who have undergone renal transplantation from living unrelated donors (LURD). These transplants were all performed in one of two centers: Bagdad, Iraq or Bombay, India. We have observed a parallel increase in the number of cases of Pneumocystis carinii pneumonia (PCP) post-renal transplant. We conducted a ten-year retrospective analysis (1986-1995) of patients who developed PCP post-renal transplant to determine the risk factors associated with the development of this infection, with particular reference to the type of transplant and the center in which the transplant was performed. Over this period, 270 renal transplant patients were followed at this hospital and 10 episodes of PCP were documented (3.7%). Six of these cases occurred within the last 2 years, as compared to only 4 cases in the preceding 8 years. All of the cases observed in the last 2 years occurred in patients who had undergone renal transplantation from LURD in Iraq or in India. During the same period, we observed no cases of PCP in patients who had undergone transplantation in Israel (cadaver or related living donor transplants). We could find no difference between patients undergoing transplant from LURD and those undergoing other transplants in terms of immuno-suppressive therapy, frequency of organ rejection episodes or coexistent CMV infection. All patients were of Arab descent and live in the West Bank. Although we cannot identify any obvious explanation for this association, we believe that these cases represent a true cluster phenomenon. We therefore feel it is warranted for all recipients of renal transplants from living unrelated donors seen in our hospital to receive prophylactic therapy for Pneumocystis carinii pneumonia.