This review describes the use of fish oil in the treatment of patients with immunoglobulin (Ig) A nephropathy. IgA nephropathy is the most common glomerular disease worldwide. It has a variable course and leads to end-stage renal disease in a substantial number of cases. Among the 4 published randomized clinical trials that tested the efficacy of fish-oil treatment of IgA nephropathy, 2 reported beneficial effects on renal function and 2 showed negative results. In the largest trial conducted by my collaborative study group, convincing evidence was provided for protection against progressive renal disease after daily treatment for 2 y with fish oil providing 1.8 g eicosapentaenoic acid and 1.2 g docosahexaenoic acid-the 2 major n-3 polyunsaturated fatty acids in fish oil. Oral prednisone has also been advocated, especially in the treatment of children with IgA nephropathy. Two randomized trials are currently underway in the United States to resolve the discrepancy of results in previous fish-oil trials and to determine whether corticosteroids or fish oil is the better treatment of patients at risk for developing progressive disease; results of these studies are not yet available.