After a 3-month, single-blind, run-in period, 151 patients with intermittent claudication were randomly allocated to receive the antiplatelet agent ticlopidine (250 mg twice per day) or an identical placebo. One hundred and twenty patients completed the double-blind phase of the trial, which lasted 21 months. The primary analysis was performed according to the "intention-to-treat principle" in all 151 enrolled patients. There was, continuing on from the third month after randomization, a progressive and sustained improvement of the pain-free and maximum walking distances in the two treatment groups that was significantly greater in the ticlopidine group. The ankle-arm systolic blood pressure ratio at rest and after exercise increased in a significant manner in the ticlopidine group only. In a secondary analysis, with exclusion of 25 patients because of protocol violations at selection, consistently significant differences in favor of the ticlopidine group were still observed for maximum walking distance and systolic ankle-arm blood pressure ratio, both at rest and after exercise. No major side effects were reported in the treated group. It is concluded that long-term treatment with ticlopidine improves walking ability and ankle systolic blood pressure in patients with claudication.