Prostate cancer is unique among the potentially lethal human malignancies in the wide discrepancy between the high prevalence of histologic changes recognizable as cancer and the much lower prevalence of the clinical disease. Despite the availability of effective tests for early detection and of effective treatment for cancers so detected, the diagnosis usually is not established until the tumor is locally advanced or metastatic. Yet, physicians hesitate to use these tests for fear that many cancers found would be latent, of little threat to the life or health of the host, and treatment could introduce inappropriate morbidity. Latent or "clinically unimportant" cancers can be distinguished from those that are clinically important by the larger volume, higher grade, and greater invasiveness of the latter. The available tests can detect only those cancers large enough to be palpable, visible on ultrasound, or capable of elevating the serum level of prostate-specific antigen. Such cancers are clinically important and should be treated for cure if the life expectancy of the patient is sufficiently long and the morbidity rate of therapy is low. Early detection of prostate cancer using the tests that are available today may widen the window of opportunity so that treatment indeed becomes possible in those for whom it is necessary.