A total of 17 patients who had undergone transurethral (16) or open (1) enucleation of the prostate for presumed benign prostatic hyperplasia had prostatic adenocarcinoma: 10 on the basis of examination of the resected specimen (stage A) and 7 upon rectal examination performed 2 to 120 months after prostatectomy for benign prostatic hyperplasia (stage B). In all patients magnetic resonance imaging (MRI) of the prostate was performed before radical retropubic prostatectomy. Preoperative imaging was compared to pathological findings with respect to the presence, location and stage of singular or multiple prostatic carcinomas. Carcinomas were categorized according to the location within the prostate: whether on the right or left side, and whether in the peripheral zone (anterior, anterolateral or posterior) or the transition zone. The sensitivity of tumor detection for cancers originating in the peripheral zone was 81%. However, the sensitivity of detection decreased to 0% for tumors confined to the transition zone. Tumor staging was not compromised by previous prostatic enucleation or transurethral resection. MRI correctly identifies carcinomas originating in the peripheral zone but cannot detect those confined to the transition zone.