Telomerase is a ribonucleoprotein that synthesizes telomeric DNA on chromosomal ends. While telomerase is undetectable in most normal somatic tissues, telomerase activation has been detected by a polymerase chain reaction (PCR)-based assay (TRAP) in many immortal cell lines and various cancers, including prostate cancers. To investigate the role of telomerase in prostate cancer at the cellular level, the expression of one of the ribonucleoprotein complexes, the RNA component of human telomerase (hTR), was studied in normal, preneoplastic, and cancerous prostate tissues using a non-radioactive in situ hybridization procedure. Nine human prostates resected at the time of radical prostatectomy were studied. In each case, archival paraffin-embedded samples from normal tissue, prostatic intraepithelial neoplasia (PIN) lesions, the putative precancerous lesion, and prostate carcinomas were selected for in situ hybridization. hTR mRNA expression was detected in carcinomatous glands of seven out of the nine cancers (75 per cent). Furthermore, in seven out of the eight cases showing PIN lesions, the epithelial cells of PIN foci also expressed hTR mRNA. By contrast, in normal tissue, epithelial cells were negative, whereas hTR mRNA expression was detected in the basal cells. The detection of hTR mRNA in PIN lesions clearly strengthens the link between PIN and carcinomatous glands and suggests that telomerase expression occurs early in prostate carcinogenesis. Furthermore, this study confirms previous experimental data suggesting that the basal cell layer is the stem cell compartment in prostate.