The combination of thromboxane synthase inhibition with thromboxane receptor antagonism has been shown to result in a strong inhibition of platelet aggregation and a prolongation of the bleeding time (Gresele et al., J. Clin Invest 1987; 80: 1435-45). Ridogrel is a single molecule that efficiently achieves both inhibitions in human volunteers. The present study was performed in patients with obstructive peripheral arterial disease and elevated plasma beta-thromboglobulin levels. Patients were treated with either 2 x 300 mg ridogrel or 2 x 300 mg placebo per day for 2 1/2 days, according to a double blind randomised parallel design. Plasma beta-thromboglobulin decreased significantly throughout active treatment starting within 2 h after administration; serum and urinary immunoreactive TxB2 levels and urinary 11-dehydro-TxB2 excretion were significantly lower and serum PGE2 and 6-keto-PGF1 alpha levels significantly higher with ridogrel; no changes were observed in the placebo-treated group. In conclusion this study demonstrates a reduction of platelet activation in vivo by ridogrel.