Neoadjuvant hormonal therapy (NHT; androgen ablation) is used prior to radical prostatectomy (RP) in an attempt to pathologically "downstage" prostatic adenocarcinoma and ultimately to improve disease-free survival. This study describes the pathologic effects of NHT with the antiandrogen cyproterone acetate, 300 mg/day for 12 weeks, on the RP specimens from men with clinically localized (stage T1 or T2) prostatic adenocarcinoma. There were 101 men in the pretreatment group (CPA) and 91 men in a control group who were treated with surgery alone. The prevalence and extent of morphologic effects were recorded for the nonneoplastic prostate, high-grade prostatic intraepithelial neoplasia, and invasive adenocarcinoma. The commonest effects on the nonneoplastic prostate were atrophy and basal cell hyperplasia and prominence. High-grade prostatic intraepithelial neoplasia was more commonly identified in the surgery alone group than the CPA group (p <0.01). In the CPA group, flat and low tufted patterns of high-grade prostatic intraepithelial neoplasia predominated. Following NHT, the adenocarcinoma showed characteristic morphologic alterations, including reduction in cytoplasmic quantity, cytoplasmic vacuolation, nuclear pyknosis, reduced gland diameter, and mucinous breakdown. In many cases there was prominence of collagenous stroma, obscuring malignant glands. Compared with the surgery alone group, the CPA group RP specimens had a significantly lower mean specimen weight (40.3 g vs 46.5 g, p = 0.025) and less tumor extent by several measures. Organ-confined tumor (stage pT2, margin negative) was found in 41.6% of the CPA group compared with 19.8% of the surgery alone group (p = 0.0017). The overall rate of margin positivity was lower in the CPA group (27.7% vs 64.8%, p = 0.001). We consider that the difference in margin positivity is the result of tumor shrinkage with a decreased likelihood of sampling in routine sections. There was no significant difference in the rate of extraprostatic extension between the two groups. There was elevation of the Gleason score in the RP specimens versus baseline biopsy in 60% of the CPA group compared with 33% of the surgery alone group (p = 0.02). The higher rate of elevation in the CPA group largely resulted from an increase in primary or secondary Gleason score 5 tumor, a morphologic artifact introduced by NHT. Because of this, we recommend not giving a Gleason grade to RP specimens following NHT. Monotherapy with CPA has similar pathologic effects on benign and malignant prostate tissue as does dual agent androgen blockade. Prolonged follow-up of these patients is required to determine if NHT with CPA leads to improved disease-free survival.