Recently, the cDNA encoding a novel candidate for prostate cancerspecific antigen, named prostate-specific membrane antigen (PSM), was cloned from the LNCaP prostate cancer cell line (R. S. Israeli, C. T. Powell, W. R. Fair, and W. D. W. Heston, Cancer Res., 53: 227–230, 1993). More recently, they also identified an alternatively spliced variant of PSM in normal prostate tissues (S. L. Su, I-P. Huang, W. R. Fair, C. T. Powell, and W. D. W. Heston, Cancer Res., 55: 1441–1443, 1995). The cDNA of this variant, named PSM', lacks 266 nucleotides present in PSM cDNA, so the transcripts derived from this particular nucleotide sequence can be regarded as PSM-specific transcripts. In this study, we investigated the expression of PSM-specific transcripts in 15 specimens of prostate cancer obtained by needle biopsy using in situ hybridization with a newly developed RNA probe. PSM-specific transcripts were detected in most of the carcinoma cells in all of the specimens examined, and the level of expression was higher in carcinoma cells from hormone-refractory patients than in the cells of those who showed a good response to hormonal therapy. In addition, increased expression of PSM-specific transcripts was also associated with an increased Gleason score. In the normal prostate, on the other hand, PSM-specific transcripts were limited to the basal cells of the prostate glands. These results clearly show that expression of PSM-specific transcripts is closely associated with malignant transformation of the prostate; thus, in situ hybridization for detection of the transcripts is useful for the diagnosis of prostate cancer.