In a pediatric and in an adult group of patients with hematuria and normal creatinine clearance overnight urine examination was carried out on 2 nonconsecutive days by means of phase contrast microscopy by two independent observers working in two different institutions. In this way it was possible to distinguish between patients on the basis of dysmorphic (glomerular) and isomorphic (nonglomerular) red cells in urine and to correlate the findings with the final diagnosis. A clear-cut indication (more than 80% of isomorphic and/or dysmorphic red cells) was obtained in 163 patients (102 of pediatric age) and final diagnosis of hematuria correlated with red-cell microscopy findings in 96.4% of glomerular diseases and in all cases of nonglomerular origin. Mixed hematuria (50-75% of dysmorphic red cells) was found in 2 cases of renal tuberculosis, 2 cases of polycystic kidney disease and in 1 child with viral meningoencephalitis with a bladder stone. The data indicate that the method is safe and accurate but further experience must be gathered for the many etiologies of glomerular and nonglomerular diseases hitherto not studied.