Mesangial proliferative glomerulonephritis without IgA deposition (non-IgA MesPGN) is commonly detected in biopsy specimens, but the clinicopathological correlation with the long-term prognosis still remains obscure. The aim of our study is to elucidate the long-term prognosis and the clinicopathological prognostic factors in patients with non-IgA MesPGN. We mailed questionnaires to 122 patients with primary glomerulonephritis who were biopsied between 1963 and 1975. Information was obtained from 109 of these 122 patients and 55 were histologically rediagnosed as having non-IgA MesPGN. The histological alterations of glomeruli and tubulointerstitium were classified into five grades. The mean period between the biopsy and the questionnaires was 20.5 years. Six of the 55 patients with non-IgA MesPGN developed end-stage renal failure and histopathological alterations of renal biopsies from these six patients were classified into grade IV or V. The presence of hypertension, heavy proteinuria of over 2+ or renal insufficiency at the biopsy was related to the severe histological changes, a grade of IV or V and to a poor prognosis. The renal survival rate of all the 55 patients was 88.3% at 20 years after the biopsy, while that of the 12 patients with severe histological changes was 48.6%. Although non-IgA MesPGN is considered to be a heterogeneous disease, we cannot ignore the incidence of this disease and thus consider it to be one of the important primary glomerulonephritides that occur in childhood.