The effect of a short course of anti-platelet agents, started preoperatively, on the patency of femorodistal bypass grafts is unknown. One hundred and forty-eight such grafts were randomized to act as controls or to receive dipyridamole 200 mg b.d. for 48 h pre-operatively and dipyridamole 200 mg b.d. with aspirin 300 mg daily for 6 weeks after surgery. Patients were well-matched and the mean pre-operative pressure index of 0.37 rose to 0.78 during the first postoperative week. No deaths were attributable to the treatment. Ninety-three grafts were autogenous vein and the remainder were prosthetic (GORE-TEX (PTFE), human umbilical vein or externally supported Dacron). At 1 year autogenous vein cumulative patency was 75 per cent. Overall results showed higher patency in the treated group (P = 0.012) which was entirely accounted for by the difference between prosthetic dipyridamole and aspirin group (85 per cent patency) and prosthetic control groups (53 per cent patency, P = 0.005) and arose during the first postoperative month. There were 11 deaths and 8 amputations in the dipyridamole and aspirin group and 8 deaths and 12 amputations in the control group. It is concluded that a six week perioperative course of dipyridamole and aspirin allows the patency of prosthetic femorodistal bypass to approach that of autogenous vein, and the regimen therefore is recommended for patients who may require a prosthetic graft.