A review of the histologic findings in 27 patients originally classified as having some form of membranoproliferative glomerulonephritis (MPGN) revealed 13 with MPGN Type I and 14 with dense deposit disease (DDD). In all 14 cases where electron microscopy was performed, the histologic diagnosis was confirmed. In nine cases the diagnosis of DDD was easily made in histologic sections on the basis of ribbon-like, brightly PAS positive thickening of the GBM, without "splitting" and with relatively slight mesangial proliferation. However, in five cases the picture closely resembled MPGN Type I, with hypercellularity, "splitting" and only focal ribbon-like thickening of the GBM, which required oil immersion for recognition. There was no correlation between the serum C3 levels and the morphologic diagnosis: nine (4 MPGN Type I, 5DDD) had persistently low C3 levels, two (1 MPGN Tye I, 1DDD) were normocomplementemic, and in 16, the C3 levels varied. C3 levels increased with time in nearly all patients. The clinical course was similar in patients with MPGN Type I and DDD. Significant correlations between the rate of development of renal failure and sex, age of onset, nephrotic syndrome or therapy could not be made. The five year survival rate was 87%; 12 developed renal insufficiency by five years. Although morphologically distinct, these findings suggest that DDD is clinically indistinguishable from MPGN Type I.