The effect of fish-oil dietary supplement on the rate of progression of renal failure in 11 patients with IgA nephropathy was investigated by comparing the slope of the plot of reciprocal serum creatinine versus time before and after supplement. After supplement, the slope significantly decreased in two patients, increased in two, and was not significantly altered in the remainder. Before supplement, serum IgA, IgA circulating immune complexes, and in-vitro Ig production were not significantly different from normal controls. After supplement, urinary protein transiently increased, serum IgA circulating immune complexes decreased, and serum triglyceride and in-vitro platelet aggregation tended to decrease. Only the change in serum IgG and spontaneous in-vitro IgG production correlated with the change in the rate of renal functional deterioration. No clinical or biochemical parameters before supplement predicted the subsequent response. In patients with IgA nephropathy, non-immune factors probably contribute to progression of renal failure, and in these patients fish-oil dietary supplement has either no demonstrable or an unpredictable effect.