Platelet inhibitory drugs may be used in an attempt to reduce high failure rates in small artery bypass using prosthetic grafts. We evaluated in patients the effects of aspirin and dipyridamole (ASA/DPM) on radiolabelled platelet accumulation on femoro-popliteal grafts of Dacron, polytetrafluorethylene and saphenous vein. Forty-seven patients awaiting femoro-popliteal bypass received, at random and double blind, either ASA/DPM or placebo capsules. On the seventh postoperative day, the 42 remaining patients with patent grafts were injected with autologous 111-Indium labelled platelets. Graft platelet accumulation was expressed as the Thrombogenicity Index (TI), which was defined as the mean daily rise in the ratio of radioactivity over the graft to that over the contralateral thigh. ASA/DPM reduced mean (+/- s.e. mean) TI in Dacron grafts from 0.25 +/- 0.09 on placebo to 0.16 +/- 0.05 (p less than 0.05). Mean TI in polytetrafluorethylene grafts was also significantly lower at 0.06 +/- 0.01 on ASA/DPM compared to 0.16 +/- 0.03 on placebo. Vein grafts, however, accumulated few, if any, labelled platelets and ASA/DPM had no further influence. ASA/DPM reduces platelet accumulation in prosthetic femoro-popliteal grafts in man.