Platelet aggregation using a single platelet counting technique in whole blood, was determined on 18 patients with primary Raynaud's phenomenon and 17 age-matched controls. Platelet aggregation in the Raynaud's patients was also assessed during a double-blind, crossover trial to investigate the efficacy of the angiotensin converting enzyme (ACE) inhibitor, enalapril. There were no differences in platelet aggregation to collagen, arachidonic acid, ADP or PAF, or in plasma levels of beta-thromboglobulin (BTG), platelet factor 4 (PF4) or thromboxane B2(TxB2) between the Raynaud's group and the normal controls. Similarly, there were no differences in these parameters in the Raynaud's group during treatment with enalapril when compared to placebo. It is concluded that patients with primary Raynaud's phenomenon have no evidence of abnormal platelet aggregation or increased platelet activation, and that platelet aggregation is not affected by enalapril.