Nucleolar organizer regions (NOR) are DNA loops encoding ribosomal RNA production. Detectable by the argyrophilia (AgNOR) of associated proteins, AgNOR numbers correlate with growth fraction and may have diagnostic and prognostic utility in human tumors. Because nucleolar size is important in the diagnosis of prostate carcinoma, we compared AgNOR counts in benign, atypical, and malignant prostate lesions and correlated them with nucleolar diameter. We counted AgNOR in benign prostatic hyperplasia, atypical adenomatous hyperplasia, intraductal dysplasias, and carcinomas of various Gleason grades. The mean AgNOR count per nucleus for benign prostatic hyperplasia nuclei was 4.51; for atypical adenomatous hyperplasia, 5.64; for intraductal dysplasias, 7.35; for carcinoma of Gleason grades 2 + 3, 8.87; and for carcinoma Gleason grades 4 + 5, 10.42. Counts in the carcinomas and intraductal dysplasias were significantly different from those of benign prostatic hyperplasia. Mean AgNOR counts of the carcinomas and intraductal dysplasias did not overlap with those of benign prostatic hyperplasia, suggesting that intraductal dysplasia shares more with carcinoma than benign prostatic hyperplasia. Nucleolar diameters increased from benign prostatic hyperplasia to atypical adenomatous hyperplasia, intraductal dysplasias, and the carcinomas, correlating with increasing AgNOR counts. Nucleolar diameters in the carcinomas were significantly different than those of benign prostatic hyperplasia; those in intraductal dysplasias were not. Our findings suggest that AgNOR counts are superior to nucleolar diameters alone in separating intraductal dysplasias from benign prostatic hyperplasia but provide no additional information in diagnosing carcinoma.