Treatment with nonsteroidal anti-inflammatory drugs (NSAIDs) was first used in glomerulonephritis (GN) in 1966 but its efficiency is still debated. We studied the antiproteinuric effect of such a treatment in a double-blind study. 29 GN patients with normal renal function (17 membranoproliferative GN, 12 IgA GN) were randomly assigned to receive 100 mg/day of diclofenac or placebo for at least 2 months. There was a significant antiproteinuric effect of diclofenac versus placebo with a fall of 70% in the diclofenac group versus 6% in the placebo group (p less than 0.001 with the Mann-Whitney test). The median was 3 mg/min at onset and 2.45 mg/min after 2 months treatment with the placebo. In the diclofenac group, it was 2.2 and 0.95 mg/min, respectively (p less than 0.01). Diclofenac did not significantly increase creatinine levels. Gastric irritation was noted only once. This study establishes the short-term antiproteinuric action of diclofenac. Whether this action affects the final outcome is not yet determined.