Random systematic ultrasonographically guided transrectal core biopsies of the prostate were performed in 73 patients with pulmonary malignancies to exclude prostate cancer as a primary malignant neoplasm. Of the patients 41 had normal prostates as judged by digital rectal examination, 27 had firm prostates and 5 had clinical stage B nodules. Hypoechoic areas were seen on transrectal ultrasonography in 14 of the 41 patients (34%) with normal prostates. Biopsy of the hypoechoic areas in this subgroup detected only 1 grade II prostate cancer. In another patient with normal transrectal ultrasound grade I cancer was detected by mapping of the prostate with 6 systematic ultrasonographically guided transrectal core biopsies. Of the remaining 39 patients with normal prostates transrectal ultrasound detected no hypoechoic defect in 26, a specificity for the detection of prostate cancer of 67%. Multiple core biopsies revealed prostate cancer in 15 of the 32 cases of palpably abnormal prostates, including 12 that were hypoechoic. Prostate cancer is a rare histopathological finding in men with normal prostates on rectal examination. Transrectal ultrasound detected only 1 of 2 low volume prostate cancers in our study group. Thus, ultrasound seems to have little use in patients with prostates that appear normal on digital examination, and its specificity is low.