Eighty patients had idiopathic renal hematuria and normal renal function. Renal biopsy showed minimal changes or focal glomerulonephritis in two thirds of the patients. The remainder exhibited diffuse proliferative glomerulonephritis and included nine patients with segmental glomerular sclerosis. Electron microscopy disclosed alterations of the capillary loop in 23 biopsy specimens and electron-dense deposits in 11. Immunofluorescent microscopy identified glomerularbound immunoglobulins, C3, or fibrinogen in 58% in a generalized distribution. IgG was the immunoglobulin seen most commonly. IgA and IgM were present in 14 and 13 biopsy specimens, respectively. These findings suggest that idiopathic renal hematuria is a clinical syndrome rather than a single disease with varying underlying renal findings. Both immunologic and nonimmunologic mechanisms may be involved, and the prognosis is favorable even in patients followed up for 11 years.