We have studied haemostatic parameters in 12 patients with Cushing's syndrome. Three patients had prolonged bleeding times, and in all seven patients whose bleeding times were measured 3-6 months after surgical treatment the postoperative bleeding times were shorter (mean 7.5 min) than the pretreatment times (mean 12.3 min). In ADP- or adrenaline-induced aggregation the second wave was lacking in six and the degree of aggregation was borderline or subnormal in five patients. One patient had, in addition, a severe defect in collagen-induced aggregation. However, thromboxane B2 production of the platelets from both endogenous and exogenous arachidonic acid was unaffected. Factor VIII:C, RAg and Rcof activities were all elevated, and in patients with severe disease F VIIIR:Ag and F VIII:Rcof activities were markedly more elevated than F VIII:C activity. The changes in both primary haemostasis and in factor VIII activities correlated clearly with the clinical severity of the disease.