Adrenoleukodystrophy is an inherited, progressive disorder of the CNS white matter and adrenal glands, associated with the pathognomonic accumulation of saturated very long-chain fatty acids, particularly C26:0. It has been previously demonstrated that the fatty acids that accumulate in adrenoleukodystrophy are, at least in part, of dietary origin. This observation, coupled with success of dietary phytanic acid restriction in a related disorder, Refsum's disease, encouraged us to develop a diet that would restrict dietary C26:0 intake. We report here the very long-chain fatty acids content of 135 common foods and development of a diet that restricts C26:0 intake to 3 mg, compared to 12 to 40 mg in the standard American diet. To limit C26:0 intakes it was found necessary to restrict fatty foods and the outer coverings of vegetables and fruits. In contrast to the success of phytanic acid restriction in limiting disease progress in Refsum's patients, administration of the very long-chain fatty acid-restricted diet to seven adrenoleukodystrophy patients for 3- to 24-month periods was found to be ineffective in lowering their plasma very long-chain fatty acids or in improving clinical status. Recently endogenous synthesis of C26:0 has been demonstrated and this may account for the failure of dietary therapy in adrenoleukodystrophy. It is possible that dietary restriction may augment other therapies in the future.