An unselected population of 8,954 children, age 8 to 15 years, was screened for hematuria. Four urine specimens from each were examined; microscopic hematuria was found in one or more specimens in 4.1%, and in two or more specimens in 1.1% of the children. The prevalence was not age or sex dependent. Those with two or more positive samples were re-examined twice during a half-year period: 33 had hematuria of 6 or more RBC/0.9 mm3, or more than 100,000 RBC/hour, on both occasions; renal biopsy performed on 28 of them revealed two cases of IgA-IgG nephropathy, one of focal segmental sclerosis, one of extracapillary glomerulonephritis, and one of possible hereditary nephritis. In 12 patients the biopsy was entirely normal; the rest showed equivocal changes. Co-existing proteinuria and the degree of hematuria correlated well with the severity of the morphologic alterations. Pathologic findings in microscopic hematuria seem to be less frequent than in hematuria in general; in most such patients, renal biopsy is probably not indicated. In some children the low-grade hematuria may merely represent the upper end of physiologic variation.