Organ transplant recipients are at an increased risk of developing malignancies due to prolonged immunosuppression. However, the rate and clinical course of neuroendocrine tumors (NETs) following organ transplantation has not been assessed so far. We have retrospectively analyzed patients undergoing organ transplantation between 1985 and 2001 in order to assess the frequency and clinical course of NETs in organ transplant recipients. 3,190 organ transplant recipients with sufficient clinical data were identified (2,521 kidney and 669 heart transplants). In total, 161/3,190 patients (5%) developed malignancies, with 6 of them being classified as NETs (0.18%). Interestingly, all 6 patients were diagnosed with undifferentiated neuroendocrine carcinomas, while no indolent NETs were seen. Four of these patients had undergone renal, 1 patient heart and 1 patient both heart and renal transplantation. All 6 patients were given chemotherapy, but none of them responded, as all patients showed disease progression after a median of 3 cycles of chemotherapy (range 1-4) with the median survival being 4.8 months (range 2-11). The occurrence of NETs/undifferentiated neuroendocrine carcinomas following organ transplantation appears to be rare, with an incidence comparable with the normal population. Our data suggest a highly aggressive course with a dismal prognosis and unresponsiveness to chemotherapy.