Fifty children had benign familial hematuria. They were from 43 families showing neither deafness, heavy proteinuria, nor chronic renal failure and had a nonprogressive course. Light microscopy of renal biopsy specimens showed little or no glomerular changes. Immunofluorescence showed no significant glomerular deposits of immunoglobulins or complement components, but deposition of C3 in the arteriolar walls was observed in 21 of the 39 patients examined. Electron microscopy demonstrated widespread attenuation of the glomerular basement membrane (GBM) in 19 patients, focal attenuation in 22, and normal GBM in nine. These observations suggest that patients with benign familial hematuria are heterogeneous and that the thin GBM may be related to hematuria.