We evaluated neurologic function in 18 patients, ages 5 to 26 years, with cystic fibrosis. Eight were deficient in vitamin E. Sural nerve conduction latency was increased and nerve action potential amplitude decreased in the vitamin E-deficient group in comparison with the vitamin E-sufficient group. Two vitamin E-deficient patients had absent deep tendon reflexes; findings of clinical neurologic examinations were otherwise normal. We recommend early supplementation with vitamin E for patients with cystic fibrosis who have pancreatic insufficiency, to prevent neurologic dysfunction.