We have devised an analytical method for the determination of fatty acid composition of erythrocyte membrane sphingomyelin by chemical ionization mass spectrometry combined with capillary column gas‐liquid chromatography. Fatty acid composition of erythrocyte membrane sphingomyelin from 8 patients with adrenoleukodystrophy (ALD) and 16 healthy controls were examined by this method. The ratio of hexacosanoic acid (C_26.0) to docosanoic acid (C_22:0) in erythrocyte membrane sphingomyelin from ALD patients was 2.6‐fold higher than that of the controls. This result suggests that biochemical diagnosis of ALD is possible by the analysis of fatty acid composition of erythrocyte membrane sphingomyelin. Furthermore, it demonstrates that biochemical abnormality in ALD is the generalized abnormal metabolism of very long‐chain saturated fatty acids.