Platelet activation may have a pathophysiological role in Raynaud's phenomenon (RP). However, previous studies have shown conflicting results. This may be related to patient selection and the choice of platelet function assay. In this study, we have assessed platelet function of 30 patients with severe RP with (n = 14) or without (n = 16) systemic sclerosis (SSc), using a whole blood platelet aggregation (PA) assay. Raynaud's medication or other drugs which may affect PA were stopped at least 2 weeks previously. Spontaneous whole blood PA and that induced by 0.5 microM adenosine diphosphate and 0.6 and 1 microgram/ml collagen were significantly increased in both groups of patients when compared with controls. There were no significant differences in PA between the 2 groups of patients. Using a more physiological assay, patients with severe RP, whether or not associated with SSc, were shown to have abnormally increased platelet activity. Hyperactive platelets may further impede blood flow in RP.