Prostate cancer is usually heterogeneous and multifocal, with diverse clinical and morphologic manifestations. Current understanding of the molecular basis for this heterogeneity is limited, particularly for prostatic intraepithelial neoplasia (PIN), the only putative precursor which can be identified according to morphologic criteria. However, it is likely that prostatic adenocarcinoma might arise from precursor lesions other than PIN, although these cannot be recognized with certainty at the present time. In this review, we summarize the current state of knowledge regarding the cell-biological and genetic bases for linking PIN and prostatic adenocarcinoma. It is conceivable that a stem cell of basal phenotype, or an amplifying cell, is the target of prostatic carcinogenesis. Prominent genetic heterogeneity is characteristic of both PIN and carcinoma; and multiple foci of PIN arise independently within the same prostate. This observation suggests that a field effect probably underlies prostatic neoplasia. Multiple foci of cancer also often arise independently, lending additional support to this hypothesis. The strong genetic similarities between PIN and cancer strongly suggest that evolution and clonal expansion of PIN, or other precursor lesions, may account for the multifocal etiology of carcinoma. Uncertainties with respect to identification of those precursor lesions which are most likely to progress to invasive and metastatic prostate cancer reinforce the requirement for objective immunohistochemical or molecular biological markers of the aggressive phenotype.