Asymptomatic microhematuria is a common finding, occurring in 13% of adult men and postmenopausal women in Rochester, Minn. Previous recommendations to perform cystoscopic and excretory urographic examinations on all patients with this finding were based on findings in referred patients. In the present population-based study, the frequency of serious urologic disease in patients with asymptomatic microhematuria was 2.3%; only 0.5% had bladder or renal cell carcinoma. Urologic malignant lesions occurred more frequently in the elderly. There was a trend toward more serious diseases in those with higher grades of hematuria. Complete urologic investigation of all patients with any degree of asymptomatic microhematuria cannot be recommended. The presence of other risk factors should be considered in opting for further evaluation.