Individuals with Noonan's syndrome are likely to have one or more coagulation abnormalities: complex platelet function defects, partial Factor XI deficiency, or von Willebrand's disease. A distinctive platelet function defect has not been identified. The authors describe a 24-year-old woman with Noonan's syndrome, chronic idiopathic thrombocytopenic purpura (ITP), and a platelet function defect characterized by a >15-minute bleeding time, failure of aggregation and release with 10 μM ADP, 10 nM epinephrine, 750 μM arachidonic acid or 0.019 g/L collagen. Amixture of aspirin-treated platelets with the patient's platelets failed to correct the defect. Addition of 2.5 μM U46619 (a PGG2 analogue) corrected the aggregation and release defect. An electron microscopic analysis failed to reveal structural abnormalities. Thus, the platelet function defect in this patient appears to be a functional deficiency of cyclooxygenase. The presence of autoantiplatelet antibodies in a clinical setting consistent with chronic ITP raises the possibility that the defect may be acquired.