The discrepancy between the high prevalence of prostate cancer found at autopsy and the low incidence of clinical cancer prompted a study to determine whether the new diagnostic tests, that is ultrasonography and serum prostatic specific antigen (PSA) levels, detect prostate cancer at an earlier stage than the traditional test, digital rectal examination, without detecting a larger proportion of clinically unimportant cancer. Clinically detected cancer treated by radical prostatectomy (306 cases) and incidental cancer found in cystoprostatectomy specimens (90) were categorized into 3 groups by the volume, grade, extent of the cancer and outcome of treatment: clinically unimportant tumor (0.5 cm.3 or less, Gleason grades 1 to 3 and confined to the prostate), clinically important curable cancer (more than 0.5 cm.3 or grade 4 or 5 and confined, or with microscopic extracapsular extension) or advanced disease (extensive extracapsular extension, seminal vesicle invasion or lymph node metastases). Of 306 clinically detected tumors 9% were unimportant and 29% were advanced. In contrast, incidental cystoprostatectomy disease was either unimportant (78%) or curable (22%) and no tumor was advanced (p < 0.0005). Cancer detectable by digital rectal examination, ultrasonography or PSA was distributed similarly among the 3 groups. Impalpable cancer detected by PSA was less likely to be advanced (11%) than cancer detected by digital rectal examination (34%, p = 0.01) but no more likely to be unimportant (13% versus 8%). Of 29 tumors detected only by systematic biopsies because of an elevated PSA level only 4% were advanced, while 17% were unimportant. Cancer detectable with each of the available diagnostic tests was similar and differed distinctly from that found incidentally in cystoprostatectomy specimens. The detection of impalpable cancer by PSA or ultrasound decreased the proportion of advanced tumor detected without increasing significantly the detection of unimportant disease.