When constructing arteriovenous fistulas for haemodialysis in chronic renal failure patients, one of the main problems is still their clotting tendency. Ticlopidine is an effective inhibitor of platelet aggregation. In this randomized double-blind study placebo or ticlopidine 250 mg twice daily was given to chronic uremic patients up to 4 weeks after construction of an arteriovenous fistula. 42 patients were recruited and 36 completed the trial. The fistula clotted in 8 patients on placebo and in 2 patients on ticlopidine. The difference is significant. This effect was achieved without an increased frequency of side effects compared with placebo. It is concluded that ticlopidine has a function as a thromboprophylactic drug in chronic uremic patients.