12 children with cystic fibrosis (C.F.) had lower levels of the essential fatty acid, linoleic acid, in plasma and in red blood cells, than did control children, and production of prostaglandin F2alpha (P.G.F2alpha) was higher than in controls. After 10 months of oral linoleic-acid supplementation in 6 of the children with C.F., the linoleic-acid levels in plasma, red cells, and platelets were higher and P.G.F2alpha production was lower than in the 6 children with C.F. who received a placebo lipid. Prostaglandin F2alpha is associated with bronchoconstriction and its increased production in children with C.F. might be causally related to their chronic pulmonary disease.