Proteinuria has been shown to be an adverse prognostic factor in IgA nephropathy. The benefit of achieving a partial remission of proteinuria, however, has not been well described. We studied 542 patients with biopsy-proven primary IgA nephropathy in the Toronto Glomerulonephritis Registry and found that glomerular filtration rate (GFR) declined at Ϫ0.38 Ϯ 0.61 ml/min per 1.73 m 2 /mo overall, with 30% of subjects reaching end-stage renal disease. Multivariate analysis revealed that proteinuria during follow-up was the most important predictor of the rate of GFR decline. Among the 171 patients with Ͻ1 g/d of sustained proteinuria, the rate of decline was 90% slower than the mean rate. The rate of decline increased with the amount of proteinuria, such that those with sustained proteinuria Ͼ3 g/d (n ϭ 121) lost renal function 25-fold faster than those with Ͻ1 g/d. Patients who presented with Ն3 g/d who achieved a partial remission (Ͻ1 g/d) had a similar course to patients who had Յ1 g/d throughout, and fared far better than patients who never achieved remission. These results underscore the relationship between proteinuria and prognosis in IgA nephropathy and establish the importance of remission.