A study was performed to evaluate the effect of low-dose aspirin (75 mg/day) on platelet reactivity in patients with peripheral vascular disease. Platelet function was measured in 31 patients with advanced peripheral vascular disease requiring surgery. Shear-induced haemostasis and collagen-induced thrombus formation were used as indicators of platelet reactivity. They were measured in non-anticoagulated fresh whole blood samples ex vivo using a haemostatometer. Results were compared with those from 30 age- and sex-matched controls. Shear-induced haemostasis and collagen-induced thrombus formation were also measured in 18 of the 31 patients before the start of aspirin administration (before operation) and I week after surgery. Patients with advanced peripheral vascular disease were found to have increased platelet reactivity with a mean(s.e.m.) shear-induced haemostasis index of 1977(180) mmHgs compared with a control value of 3161(234) mmHg s (P < 0.001) and a mean(s.e.m.) collagen-induced thrombus formation index of 3980(460) mmHg s compared with a control value of 5350(420) mmHg s (P = 0.02). Perioperative low-dose aspirin failed to inhibit platelet function in patients with peripheral vascular disease.