A unique sonographic appearance that correlates with comedocarcinoma of the prostate is described. The hard-copy and videotaped transrectal sonograms obtained in 27 patients before radical prostatectomy were reviewed. In three, the sonograms had a distinctive appearance characterized by a hypoechoic lesion containing stippled areas of hyperechogenicity. This was not present in the other 24. The sonograms of all 27 patients were compared with whole-mount histopathologic sections of the radical prostatectomy specimens. This comparison showed that the distinctive sonographic abnormalities in these three patients were caused by foci of comedocarcinoma, an aggressive variant of prostatic carcinoma (Gleason pattern 5). In none of the other 24 patients were similar sonographic findings seen and none had comedocarcinoma at histopathologic examination. These findings suggest that a hypoechoic lesion containing diffuse, stippled echogenicity may be a sonographic sign of comedocarcinoma.