In most tissues, the insulin-like growth factor II gene (IGF-II) demonstrates imprinting, being expressed exclusively from the paternal allele. Recently, a loss of IGF-II imprinting (i.e., biallelic expression) has been found in sporadic Wilm's tumors and lung carcinomas, and this molecular event may contribute to the pathogenesis of these tumors. Here, we report that in prostates removed at radical surgery for localized adenocarcinoma, both the cancer and the associated normal peripheral zone tissue have a pronounced biallelic expression of the IGF-II gene. However, this pattern of gene expression is uncommon in periurethral samples of benign prostatic hyperplasia (BPH) from the same specimens. We analyzed the status of genomic imprinting at the IGF-II locus in prostate specimens removed for carcinoma using an ApaI polymorphism in the 3' untranslated exon of the IGF-II gene. First-strand cDNA synthesis and subsequent PCR amplification were performed on 13 of 35 radical prostatectomy specimens found to be informative for analysis of allele-specific expression. Biallelic expression for IGF-II RNA was demonstrated in 10 (83%) of 12 tumor samples and 8 (73%) of 11 matched peripheral zone prostate samples but in only 2 (18%) of 11 BPH samples. RNA transcripts were readily demonstrated by Northern blot analysis, and differences in expression were not noted among normal, BPH, and tumor prostate tissues. In situ hybridization revealed production of IGF-II by both the epithelium and stroma. The finding of a frequent biallelic expression of IGF-II in peripheral prostate specimens suggests a regional pattern of IGF-II gene regulation exists in prostate tissue. We hypothesize that this tissue-specific pattern of gene expression may participate in the marked predilection of peripheral prostatic tissue for the development of carcinogenesis.