The status of the basement membrane in prostatic intraepithelial neoplasia (PIN) and adenocarcinoma is unsettled. Previous studies using antibodies directed against Type IV collagen have been hindered by intense staining around the stromal smooth muscle fibers, making interpretation of acinar staining difficult. We employed a monoclonal antibody to heparan sulfate proteoglycan (HSPG) to overcome this problem, recognizing that HSPG is present in the basement membrane of epithelial and endothelial cells, but not stromal smooth muscle cells. In 22 totally-embedded whole mount radical prostatectomies for adenocarcinoma which contained PIN, intense HSPG immunoreactivity was observed in the basement membrane of all normal and hyperplastic acini, 98% of acini with high grade PIN, and 100% of acini of well differentiated (Gleason score 5) adenocarcinoma; vessels served as the internal positive control, with consistent staining throughout each specimen. The extent of HSPG immunoreactivity in cancer decreased with increasing Gleason grade (measured as percent of acini staining, in 10% increments; p = 0.002). These findings indicate that HSPG is a consistent component of the basement membrane of benign, hyperplastic, and early neoplastic prostatic acini, and, unlike other extracellular matrix proteins such as type IV collagen, is not hindered by background staining around stromal smooth muscle cells. High grade PIN and well differentiated adenocarcinoma usually maintain an intact basement membrane, and loss of the basement membrane occurs with histologic dedifferentiation.