Fourteen patients with chronic proliferative glomerulonephritis were given for the period of one year 400 mg acetylsalicylic acid and 225 mg dipyridamole per day. During this treatment the thrombocyte aggregation became normal, however, the mean reduction of antiheparin plasma activity was not statistically significant. Normal synthesis of renal prostacyclin declined significantly as a result of treatment, while the renal thromboxane A2 synthesis remained normal even during treatment. Treatment did not influence proteinuria. The mean annual decline of glomerular filtration was greater during the investigation period than the mean annual decline in previous years, the difference was, however, only at the borderline of statistical significance. The authors did not prove a favourable effect of this treatment in patients with chronic proliferative glomerulonephritis.