Phytanic acid is a metabolite of the chlorophyll molecule and is converted from phytol, the side chain of chlorophyll. It is a saturated 20-carbon branched-chain fatty acid that can only be obtained from dietary sources. Phytanic acid oxidation (Alpha oxidation) occurs in the peroxisomes of humans. The presence of phytanic acid in tissues and plasma has been considered diagnostic of heredopathia atactica polyneuritiformis. Phytanic acid induced the adipocyte differentiation of 3T3-L1 cells in culture as assessed by accumulation of lipid droplets and induction of the aP2 mRNA marker. In two Norwegian patients, serum phytanic acid has been brought down to normal levels and one of them has been followed for 15 years.