Expression of epidermal growth factor receptor (EGFR) and ERBB2 oncoprotein were studied in paraffin-embedded normal (n = 2), hyperplastic (n = 17), and malignant (n = 147) prostatic tissues by immunohistochemistry. Strong immunoreactivity was detected in the epithelial cells of all normal and hyperplastic prostates with a new EGFR antibody (Mab31G7). In prostatic carcinomas, the EGFR immunoreactivity was variable with 47% showing uniform, 39% partial, and 14% no staining. Tumors with partial or uniform EGFR immunoreactivity were locally more advanced and of higher histological grade than the EGFR-negative tumors. EGFR-positive tumors also had two to three times higher S-phase fraction, suggesting that EGFR expression conferred proliferative advantage. Patients who had either partially or uniformly EGFR-positive carcinomas had a worse 10-yr progression-free (p = 0.05), overall (p = 0.03), and prostatic carcinoma-specific (p = 0.007) survival than those with EGFR-negative carcinomas. However, according to a multivariate analysis, EGFR did not have independent prognostic value. None of the normal, hyperplastic, or malignant prostate tissues showed clearly positive ERBB2 immunoreactivity with MAb1 antibody.