There is a 100-fold increase in the incidence of carcinomas of the vulva and anus in renal transplant recipients compared with the general population. Anogenital (anus, perianal skin, and external genitalia of both sexes) carcinomas occurred in 65 of 2150 renal transplant recipients who presented with 2298 different types of malignancy. Two-thirds of the patients were female and one-third male. They were much younger than persons with similar tumors in the general population. The average age of the females at the time of diagnosis was 37 years (range, 20-64) and of the males, 45 years (range, 34-62). The neoplasms occurred late after transplantation, an average of 88 months (range, 9-215), compared with an average of 56 months (range, 1-225.5) for all other post-transplant malignancies. The lesions involved the vulva, penis, scrotum, anus, or perianal area. Two patients also had involvement of the urethral meatus. In several female patients, there was a "field effect" with multiple tumors of the squamous epithelium of the anogenital area, vagina, or uterine cervix. Lesions ranged from in situ carcinomas (in one-third of the cases) to those with invasion of adjacent organs and lymph node metastases. Treatment varied from local excisions to radical vulvectomy, abdominoperineal resection, or penile resection, sometimes combined with excision of the inguinal lymph nodes. In several patients, there was a previous history either of condyloma acuminatum or herpes genitalis, suggesting a possible viral etiology of these tumors.