7 patients with cystic fibrosis, having serum lipids with fatty acid compositions (FACs) skewed in the direction of essential fatty acid deficiency, were given a mixture of medium chain triglycerides (MCTs) with linoleic acid-rich monoglycerides (LAMs) as food fat (about 1–1.25 g/kg body weight/day). Blood was sampled before and after a short period (4–11 days) of experimental fat feeding. The FACs of the serum triglycerides (TGs), cholesterol esters (CEs) and phospholipids (PLs) were determined before and after dietary treatment. Dietary treatment resulted in a significant increase of the previously reduced fraction of linoleic acid in all lipid classes. In TGs, the increase in the fraction of linoleic acid was accompanied by a reduction of the previously increased fraction of palmitoleic acid to normal values, by a reduction in the fraction of oleic acid to subnormal values and by an increase in the fraction of stearic acid which, however, remained normal. In PLs, the increase in the fraction of linoleic acid was accompanied by an increase in the fraction of dihomogammalinoleic acid which remained normal, however. In CEs, concomitant with the increase of the previously reduced linoleic acid fraction, there was a reduction of previously elevated palmitoleic and oleic fractions, the latter to a normal value. Thus, supplementing a MCT-containing diet with LAMs in cystic fibrosis patients results in a considerable amelioration of the previously disturbed FACs of the major serum lipid clas