Persistent microscopic hematuria is present in about 6% of the population, but probably only a small minority have hematuria that does not originate from the glomerulus. Careful analysis of phase-contrast urine microscopy by a skilled observer is critically important in the investigation of hematuria. In glomerular disease, urine microscopy often is second only to renal biopsy examination in helping make a diagnosis. Glomerular and nonglomerular hematuria are distinguished easily on phase-contrast urine microscopy or by an automated peripheral blood cell counter. However, urine microscopy provides additional information about casts and other features that may enable such disparate diagnoses as Fabry's disease, sickle cell disease, and cystine calculi to be made. Macroscopic nonglomerular hematuria is of particular significance because it is much more likely than microscopic hematuria to be associated with malignancy. Macroscopic hematuria originating from the glomerulus indicates the presence of crescentic disease, which requires urgent assessment by renal biopsy examination. We advocate a renal biopsy examination in any individual with a persisting urinary erythrocyte count greater than 100,000/mL. Thirty percent of patients with isolated microscopic hematuria have mesangial immunoglobulin A glomerulonephritis (IgAN) shown on biopsy examination and 20% to 40% of these patients will progress to renal failure without treatment.