Inactivation of the retinoblastoma (Rb) gene has been implicated in the genesis and progression of a number of tumor types, including prostatic adenocarcinomas. We have analyzed a series of 46 surgically-resected human prostatic adenocarcinomas for allelic loss of the Rb gene with PCR amplification of a highly polymorphic region of the gene. 41 of 46 tumors (89%) were informative and 11 of these (27%) had lost one Rb allele. The relative frequency of this occurrence suggests that inactivation of the retinoblastoma gene may be an important event in prostate carcinogenesis.