Analyses of serum lipids of children with cystic fibrosis (CF) have indicated a deficiency in essential fatty acids (EFA). In view of a report that intravenous administration of soybean oil emulsions normalized sweat sodium values in CF children, we studied the effects of orally administered essential fatty acids (as corn oil) for one year. Some improvement was noted in all patients, but no one consistent factor predominated. However, arachidonic acid, which was found only in trace amounts or was absent in sera of all children with CF before the clinical trial, was increased significantly in the sera after the oral administration of EFA for one year. Other findings included a significant decrease in sweat sodium and T3 in most patients.