Metabolic studies in two patients with Refsum's syndrome provide further evidence that the phytanic acid stored has a strictly exogenous origin. On a highly restricted diet the phytanic acid levels in blood and in adipose tissue fell dramatically over a period of months. This was accompanied by a number of manifestations of clinical improvement, including increase in ulnar nerve conduction velocity, increase in strength of several muscle groups, return of some previously unobtainable reflexes, improvement in pain, light touch and position sense, and improvement in objective tests of coordination. There was no improvement in vision or hearing. The results support the hypothesis that accumulation of phytanic acid plays a pathogenetic role in Refsum's disease and justify further therapeutic trials.