Of 61 children with membranoproliferative glomerulonephritis (MPGN), 6 (10%) were distinguished by segmental lesions in up to 1/3 of glomeruli. Light microscopy showed mild to moderate generalized mesangial proliferation in addition to segmental membranoproliferative lesions. Mesangial fluorescence with antiserum to C3 was present in all glomeruli while focal lesions were characterized by segmental granular fluorescence with antiserum to IgG. Segmental lesions, identified by electron microscopy in four biopsies, were produced by prominent mesangial proliferation. Subendothelial deposits were present in two. Contiguous subendothelial and subepithelial deposits were present in one, and in a fourth, capillary wall deposits could not be found although mesangial deposits were present. Circulating immune complexes were present in 2 of the 4 hypocomplementemic patients and 1 normocomplementemic patient. Clinically, all patients presented with hematuria (gross in 3) and five had proteinuria. Only one had hypoalbuminemia. All patients have improved (5 treated with alternate day prednisone) as judged by the return of complement levels to normal and by improvement in urinalysis and in glomerular morphology on subsequent biopsy. The results give evidence that focal, segmental MPGN is an early manifestation of Type 1 or, uncommonly, Type III MPGN and that the patients have an excellent prognosis.