To investigate the histopathological findings with prognostic significance in mesangial proliferative glomerulonephritis (MPGN), 52 patients with this disease were evaluated with sufficient clinical follow-up. A moderate but highly significant correlation was found between the final creatinine levels and the percentage of sclerosed glomeruli and number of atrophic tubuli per 10 high power fields (r = 0.530, p less than 0.001). When the patients with idiopathic MPGN and those with MPGN secondary to systemic or streptococcal disease were separately considered, there was a remarkable increase in the correlation index for the primary cases (r = 0.912, p less than 0.001). It was concluded that the finding with the highest predictive value in idiopathic forms is tubular atrophy. Three levels of disease were proposed, respectively defining patients with preserved renal function, established chronic renal failure and unpredictable outcome of renal function after a follow-up of five years.