Very long chain fatty acids of peripheral blood leukocytes were analyzed by gas chromatography in nine members of a family including two hemizygotes and one obligate heterozygote for adrenoleukodystrophy (ALD), as well as in twelve controls. Comparative investigations were done in cultured fibroblasts. Elevated C26:0 levels were observed in leukocytes and fibroblasts of ALD hemizygotes. The obligate heterozygote displayed a clear-cut increase of C26:0 concentration in leukocytes but not in fibroblasts. Determination of C26:0 in leukocytes may serve as diagnostic tool in the detection of ALD gene carriers.