Cystic fibrosis (C.F.) is characterised by low serum levels of essential fatty acids (E.F.A.). However, the fatty-acid pattern does not totally resemble that of dietary E.F.A. deficiency. The differences suggest a reduction in the desaturation of E.F.S. It is not known whether this defect is the primary lesion in C.F. or is the result of tissue damage in the disease. It is proposed that C.F. patients might have increased linoleic-acid requirements, and possibly specific requirements for its desaturation products.