Ultrasonography of the prostate detects some cancers that are not palpable, but the pathologic features of such cancers have not been well described. Since screening trials consistently find sonography more sensitive (though less specific) than digital rectal examination, nonpalpable cancers that are visible as hypoechoic lesions on ultrasound have been postulated to be early cancers of limited malignant potential and may not require aggressive treatment. To test this hypothesis, we determined the pathologic features and DNA ploidy value of prostate cancers in 63 radical prostatectomy specimens taken from patients with clinical stage T1 (n = 28) and T2 (n = 35) prostate cancer. In 40 patients (63%), the cancer appeared hypoechoic on ultrasound. The median volume of these cancers was 4.19 cm3 (range 0.45-19.22); 80% exhibited extra-capsular extension (ECE); 30% had seminal vesicle invasion (SVI); and 95% were nondiploid by nuclear image analysis (CAS 200 system). In patients with isoechoic cancer, tumor volume was significantly less (median 0.38 cm3) and ECE and SVI occurred less frequently (13% and 0%, respectively). Only seven (30%) had nondiploid tumors. In 35 patients, the tumor was palpable, and the pathologic features and DNA ploidy values (94% nondiploid) of these cancers were similar to those of the tumors that were visible on ultrasound. In seven patients, the cancer was visible by ultrasound but not palpable by digital rectal examination. Median tumor volume was 1.72 cm3 (range 0.45-18.98); four patients (57%) had ECE; one (14%) had SVI, and six (86%) had nondiploid cancers. We conclude that most cancers that appear hypoechoic on ultrasound are clinically important and exhibit aggressive pathologic features. Palpable cancers and sonographically visible cancers are similar and should be staged and treated similarly.