Because postmortem brain and adrenal tissue from patients with adrenoleukodystrophy (ALD) or adrenomyeloneuropathy (AMN) have been shown to contain abnormally large amounts of very long chain fatty acids (C24 through C30), we searched for such an abnormality in cultured skin fibroblasts. Total lipid extracts of cultured fibroblasts were hydrolyzed, their fatty acid composition was determined by gas-liquid chromatography, and the ratio of C26 to C22 fatty acids was calculated. In 29 control cell lines this ratio was 0.064 +/- 0.019. In 5 patients with autopsy-proved ALD the ratio was 0.778 +/- 0.139; in 6 patients with clinical features typical of ALD it was 0.764 +/- 0.092; in 2 patients with autopsy-proved AMN, 0.890 +/- 0.02; and in 2 patients with clinical features typical of AMN, 0.560 +/- 0.079. Abnormal ratios were observed in 4 of 5 ALD heterozygotes. In 3 patients in whom the diagnosis of ALD was suspected, an abnormal ratio (0.860) was observed in 1 and normal ratios (0.06 and 0.074) in the 2 others.