Of 475 renal biopsies examined by immunofluorescence, IgA was seen located selectively in the glomerular mesangium of 18 patients. These patients were generally young men and had hematuria, only minimal proteinuria, and normal renal function. Glomerular lesions consisted of focal segmental capillary hypercellularity or sclerosis and mesangial thickening due to an increase in mesangial cells, matrix, and electron-dense deposits. Both IgG and C3 were frequently seen with IgA in the mesangium as was properdin in five of six cases, but C4, a component of the classical pathway of complement activation, was seen only infrequently. Serum IgA levels were elevated in many patients, but serum complement components were normal. These findings suggest a possible unusual immune pathogenesis with local mesangial binding and alternate pathway activation of complement.