We investigated fatty alcohol metabolism in eight patients with Sjögren-Larsson syndrome, and in nine obligate heterozygotes. Fatty alcohol: nicotinamide-adenine dinucleotide oxidoreductase (FAO) activity was deficient in cultured skin fibroblasts (mean 18% of normal, n = 8) and peripheral blood leukocytes (mean 22% of normal, n = 3) from patients with Sjögren-Larsson syndrome. The palmitoyl coenzyme A-inhibitable component of FAO activity was decreased to 10% and 15% of normal in fibroblasts and leukocytes, respectively, of patients with Sjögren-Larsson syndrome. Most affected patients accumulated long-chain fatty alcohol in plasma, with a greater relative accumulation of octadecanol (mean threefold greater than normal) than hexadecanol (mean twofold greater than normal). Erythrocyte lipid alkyl ether linkages derived from hexadecanol were slightly increased in three of four patients. Fibroblasts and leukocytes from heterozygotes with Sjögren-Larsson syndrome showed mean FAO activities that were intermediate between those seen in homozygotes and in normal control subjects. The heterozygotes had normal fatty alcohol concentrations in plasma. These studies demonstrate FAO deficiency in patients with Sjögren-Larsson syndrome, and suggest that accumulation of fatty alcohol or its metabolic products may be important in the pathogenesis of this disorder.