—Recent clinical and morphological evidence established that adrenoleukodystrophy is a distinct X‐linked genetic disorder. Fatty acid compositions of lipids in the brain, adrenal and serum from seven patients were examined. Cholesterol esters of both brain and adrenal contained substantial proportions of fatty acids longer than C22 (11.8–41.9% of total in the brain and 13.4‐34.8% of total in the adrenal), while cholesterol esters from normal and pathological control specimens contained very little. These very long chain fatty acids were generally saturated in brain cholesterol esters but significant amounts of unsaturated long chain fatty acids were also present in adrenal cholesterol esters. The long chain fatty acids showed bell‐shaped distribution with C25 or C26 at the peak. Ganglio‐sides from patients’white matter also showed increased proportions of very long‐chain fatty acids, up to 50% of the total. Qualitatively similar but much milder fatty acid abnormalities were also found in galactosylceramide of the brain. On the other hand, fatty acids and fatty aldehydes of brain glycerophospholipids, adrenal free fatty acids, triglycerides and glycerophospholipids were not abnormal. Furthermore, serum cholesterol esters from two patients did not show the long‐chain fatty acid abnormality found in brain and adrenal cholesterol esters. Sequential extractions with acetone and hexane established that the characteristic birefringent material in the brain and adrenal is indeed cholesterol esters with very long chain fatty acids. This type of fatty acid abnormality has not been described in other pathological conditions and may well represent the unique biochemical abnormality that is directly related to the fundamental genetic defect underlying adrenoleukodystrophy.