Platelet sensitivity to prostacyclin was studied in 30 diabetic patients with type-II (insulin-independent) diabetes mellitus; 12 with and 18 without clinical evidence of vascular disease and in 10 healthy, normal volunteers. Platelets from diabetics, with and without peripheral vascular disease were significantly less sensitive to the antiaggregatory effects of prostacyclin as compared to platelets from normal, healthy volunteers (p less than 0.001). The findings of a decreased platelet sensitivity to prostacyclin in diabetics without vascular disease, suggests that this abnormality is a feature of diabetes mellitus, rather than a feature of established vascular disease. It may therefore be one of the factors responsible for the increased platelet aggregation that has been observed in diabetes mellitus.