Summary o 1. Hereditary ataxic polyneuritis is a rare neurological disorder which wasclinically delineated by Refsum in 1946. The demonstration by Klenk and Kahlke in 1963 that patients with the disease accumulate large amounts of 3,7,-11,15-tetramethylhexadecanoic acid (phytanic acid) in various lipid fractions has triggered intense research activities in several laboratories. 2. No endogenous synthesis of phytanic acid has been demonstrated. Thisacid stems from dietary preformed phytanic acid or phytol liberated from the chlorophyll molecule. The normal degradation of phytanic acid appears to be by a oxidation or by w oxidation followed by β oxidations. The inborn error of lipid metabolism which exists in Refsum's disease is a partial defect in the a oxidation mechanism. 3. In two patients dietary restriction of phytol/phytanic acid intake hasresulted in a reduction to normal of phytanic acid in serum, in subcutaneous fat and probably also in other body stores. At the same time there was an improvement in nerve conductivity, a return of the CSF protein concentration to normal as well as a marked improvement in the general condition and well-being of the patients. No improvement was observed in the neurological defects.