In infantile and adult Refsum's disease, the activity of phytanic acid oxidase is low in skin fibroblasts, but plasma phytanic acid levels are high. Cultured skin fibroblasts and plasma from patients with the infantile, but not the adult, disorder show marked increases in the concentration of the long-chain fatty acid, hexacosanoic acid (C26), a feature once thought pathognomonic of adrenoleukodystrophy or Zellweger's syndrome.