Results of transrectal ultrasound (TRUS) of the prostate and pathologic examination of specimens obtained at transurethral resection of the prostate (TURP) were compared in 29 patients with clinical stage A adenocarcinoma. Ten specimens contained no residual tumors larger than 5 mm in diameter; in the remaining 19 glands, 20 discrete cancers were found. At TRUS, 30 peripheral hypoechoic lesions were demonstrated, of which 11 corresponded to carcinoma at pathologic examination. Other hypoechoic peripheral zone lesions included a focal area of dilated acinar glands in 10 cases, post-TURP scarring with fingerlike projections of fibrosis in seven, dysplasia in one, and no correlation in one. Of nine tumors that were not detected prospectively at TRUS, eight were predominantly in the anterior zone and one was in the posterior peripheral zone but was isoechoic. Overall, the sensitivity of TRUS in the evaluation of clinical stage A lesions was 55% and the specificity was 37%. Clinical stage A carcinomas may be difficult to detect at US, and findings are often nonspecific. Any suspicious peripheral zone lesion should undergo biopsy with TRUS guidance before being diagnosed as malignant.