Abstract Patients with cystic fibrosis and their parents were reported to have abnormal platelet aggregation responses to prostaglandin E 1. To determine whether this is a property of the platelets, we studied the adenosine 3′:5′-cyclic monophosphate (cAMP) response of washed platelets to prostaglandin E 1. The cAMP response to prostaglandin E 1 was the same in platelets from obligate heterozygotes for cystic fibrosis and from those of healthy controls. Patients with cystic fibrosis who had deficient vitamin E levels (plasma α-tocopherol, <500 μg/dl) had significantly (p < 0.01) reduced platelet cAMP response to prostaglandin E 1 compared with patients who had sufficient vitamin E, and supplementation with water-miscible vitamin E in these patients resulted in significant increases in plasma α-tocopherol levels (p < 0.01) and in cAMP response to prostaglandin E 1 (p < 0.05). Plasma α-tocopherol levels correlated significantly with platelet cAMP response to prostaglandin E 1 in patients with cystic fibrosis (r = 0.58, p < 0.05). However, plasma α-tocopherol level was unrelated to the lymphocyte and granulocyte cAMP response to prostaglandin E 1 or to the platelet cAMP response to α 2-adrenergic stimulation. Our data suggest that patients with cystic fibrosis have no inherited defect in platelet cAMP response to prostaglandin E 1. In patients who have sufficient vitamin E, cAMP responses to prostaglandin E 1 are normal in all the formed elements of the blood.