Prostatic intraepithelial neoplasia (PIN) is a putative premalignant lesion of the prostate gland. PIN has been demonstrated to share morphologic and phenotypic similarities to invasive carcinoma of the prostate. In addition, PIN is spatially related to invasive carcinoma and occurs with greater frequency in men whose prostates harbor carcinoma. Prostate-specific antigen (PSA) is a glycoprotein produced by the prostatic epithelium. For PSA to be detected in the serum, it must traverse several tissue layers to reach the circulatory system. PSA levels associated with PIN are intermediate between those of benign and malignant prostate tissue. Spatially associated occult carcinoma, disruption of the basal cell layer, and increased vascularity may account for elevated PSA values in PIN.