It is unclear whether the changes in platelet function which are observed in systemic sclerosis are a primary characteristic of this disease or whether they occur secondary to vascular changes. Whole blood platelet aggregation was studied in 26 patients with systemic sclerosis, normal subjects matched for age, sex and secondary characteristics, 19 patients with Raynaud's disease and 19 patients with systemic lupus erythematosus. Plasma levels of fibrinogen, von Willebrand factor antigen and factor VIII:C were also measured. Systemic sclerosis was associated with a significant (P > 0.001) enhancement of the sensitivity of platelets to collagen. In contrast, significant enhancement of the response to either ADP or adrenaline was not observed. Enhanced sensitivity to collagen was not associated with the presence of either Raynaud's disease or systemic lupus erythematosus. Systemic sclerosis was associated with significantly raised levels of von Willebrand factor antigen and fibrinogen. On an individual patient basis, von Willebrand factor antigen was related to the severity of the disease whereas platelet sensitivity to collagen was not. In conclusion, this study suggests that the enhanced sensitivity to collagen which occurs in systemic sclerosis is due to a primary change in the platelet and that this change can combine with elevated levels of adhesive proteins.