AbstractPhytanic acid has been isolated from the total lipids of the liver of a human suffering from the disease, Refsum's Syndrome (Heredopathia atactica polyneuritiformis). Optical rotation measurements were made on the acid, and on the ketone 6,10,14‐trimethylpentadecan‐2‐one which resulted from the oxidative degradation of the acid and the reductive ozonolysis of phytol. From a comparison of the optical rotatory dispersion curves of the ketone and from the optical rotation data for the acid it is concluded that the phytanic acid isolated has the configuration 3‐dl 7‐d 11‐d 15 tetramethylhexadecanoic acid with the 3‐l diastereoisomer predominating.