Heredopathia atactica polyneuritiformis is a biochemically defined disease with a specific dietary treatment. It is an autosomal inborn error of metabolism. The phytanic acid is of exogenous origin and stems mainly from preformed phytanic acid in foods. In two Norwegian patients, serum phytanic acid has been brought down to normal levels and one of them has been followed for 15 years. During this period of dietary treatment there was no worsening of hearing.