The efficacy of and tolerance to ticlopidine, a platelet inhibitor, in preventing progression of obliterative arterial disease was compared with that of a placebo in a double-blind and randomized trial in 43 patients (22 on placebo, 21 on ticlopidine) over a period of one year. The course of the disease in both groups was assessed by serial angiography of the lower limbs, both at the beginning and at the end of the study. There was a significantly reduced progression in the ticlopidine group (P less than 0.01). The results support the hypothesis that platelets are of great importance in the progression of arteriosclerosis and that inhibition of platelet function is an effective principle of prevention. Ticlopidine in this respect is an alternative to the use of acetylsalicylic acid.