To evaluate the ability of transrectal ultrasonography to detect residual cancer in the prostate gland after transurethral resection in patients with stage A cancer, we studied 38 patients with stage A disease (11 stage A1 and 27 stage A2) in whom transrectal ultrasonography was done at least 3 weeks after resection. Each patient underwent radical prostatectomy, and residual cancer was present in 97% of the specimens (peripheral zone cancer in 95% and transition zone cancer in 61%). At sonography we identified hypoechoic areas suggestive of cancer in 10 patients (26%). In the pathological specimen residual cancer was present at the hypoechoic area in 8 of these cases (positive predictive value 80%). In a retrospective review of the sonograms we identified 25 hypoechoic lesions greater than 5 mm. in diameter, including 15 that corresponded to cancer in the radical prostatectomy specimens (positive predictive value 60%). Granulomas due to the transurethral resection were found in 92% of the radical prostatectomy specimens but none appeared hypoechoic on ultrasound. A total of 103 separate cancers was identified in the whole mount step sections of the radical prostatectomy specimens (2.7 cancers per patient). Of the 103 separate cancers 54 were less than 0.1 cc in volume and none of these could be identified in the retrospective review of the sonograms, 37 were 0.1 to 1.0 cc and 5 of these (14%) appeared hypoechoic, and 12 were greater than 1.0 cc and 10 of these (83%) appeared hypoechoic. Hypoechoic lesions greater than 5 mm. in diameter in the transition zone proved to be cancer in 47% of the cases, while 88% of similar lesions in the peripheral zone proved to be cancer. We conclude that suspicious-appearing hypoechoic lesions suggestive of cancer, whether in the peripheral zone or the transition zone, should be biopsied before expectant management of stage A prostate cancer is considered. Transrectal ultrasonography is useful for restaging after transurethral resection and for evaluating the extent of residual cancer in stages A1 and A2 prostate cancer.