The phytanic acid found in man stems from exogenous sources, mainly as minor parts of fish and animal fats. Free phytol, which is easily converted to phytanic acid in mammals, is present in fats of vegetable origin. Healthy individuals are able to degrade the small amounts of phytanic acid and phytol which are ingested. Accumulation of phytanic acid has been considered diagnostic for Refsum's disease, and a prerequisite for this diagnosis. However, a few patients with proven Refsum's disease have eliminated their phytanic acid stores by dietary means. Two healthy mothers of patients with Refsum's disease have been reported, in whom serum phytanic acid was considerably increased. Furthermore, phytanic acid has recently been found in patients with several socalled peroxisomal disorders (Zellweger's syndrome, neonatal adrenoleukodystrophy, infantile Refsum's disease, hyperpipecolic acidemia, rhizomelic chondrodysplasia punctata, Leber disease). Skin fibroblasts both from patients with classical Refsum's disease and from those with the peroxisomal disorders have a defect in the alpha-oxidation of phytanic acid, with a residual enzyme activity less than 10% of normal. The presence of this defect in the patients with peroxisomal disease makes it tempting to suggest that alpha-oxidation of phytanic acid normally takes place in the peroxisomes. Subcellular studies in rat liver show, however, unequivocally that the alpha-oxidation of phytanic acid is located to the mitochondria. Thus, patients with the peroxisomal syndromes must probably have a defect also in the mitochondria, in addition to the many peroxisomal deficiencies.