5 patients, including 4 with partial lipodystrophy and 1 with dense deposit disease (type II mesangiocapillary glomerulonephritis; MCGN), had positive C3 nephritic factor assays by both a crossed-gel immunoelectrophoretic and a magnesium-dependent hemolytic assay. One or more early classic complement component was significantly decreased in the serum from each patient. The titer of the C3 nephritic factor did not correlate with the presence of dense intramembranous deposits or with the severity of the renal disease. One patient with partial lipodystrophy had ultrastructural evidence suggesting transformation from type I to type II MCGN. These findings do not support a strict separation of MCGN into subtypes according to current criteria.