Tables 2 and 5 summarize the major clinical and biochemical findings in these patients. Cases 1 and 2 resemble clinically the previous cases of children reported as suffering from infantile phytanic acid storage disease, Zellweger's disease, or neonatal adrenoleucodystrophy. Cases 3 and 4 differ strikingly from these and from one another. Numerous questions remain unanswered, but it seems likely that these patients have in common defects in peroxisomal function which are related but not identical. Why some patients with phytanic acid oxidase deficiency do not have significant elevation of serum phytanic acid is not known. These results, however, make it clear that a normal serum phytanic acid level does not exclude phytanic acid oxidase deficiency. In children with a progressive neurological illness, with liver disease, retinal disease, unexplained neuropathy or deafness, detailed studies of fatty acid metabolism are indicated, including lipoproteins, serum phytanic acid, C26:C22 long-chain fatty acid ratios, serum pipecolic acid and phytanic acid oxidase levels. Electron microscopy of liver biopsy specimens should be considered. Phytanic acid oxidase may prove a useful marker for some of these illnesses, and its usefulness could extend to prenatal diagnosis and assist in genetic counselling.