Monounsaturated fatty acids, especially oleic acid (C18:1), decreased the content of saturated very-long-chain fatty acids (VLFA) in cultured skin fibroblasts from patients with adrenoleukodystrophy (ALD) and controls. When confluent ALD fibroblasts were incubated with oleic acid for 5 days in lipid-free medium, which eliminates uptake of exogenous VLFA, the mean cell content of C26:0 was decreased by 33.7 ± 10.1%. Oleic acid inhibited C26:0 synthesis in ALD fibroblasts by 58% and total fatty acid synthesis by 68 to 78%. Therefore, the elevated C26:0 levels in ALD cells may be lowered by inhibiting fatty acid biosynthesis, and inhibition of saturated VLFA synthesis by oleate may be useful in treating ALD.