Aceruloplasminemia is a newly recognized autosomal recessive disorder of iron metabolism that causes neurodegeneration of the retina and basal ganglia as well as diabetes mellitus. Our previous studies suggested that increased susceptibility to plasma lipid peroxidation secondary to iron accumulation may contribute to the pathogenesis in this disease. We now have identified increases in the very long-chain fatty acids cis-17-hexacosenoic (C26:1) and hexacosanoic (C26:0) acid in the erythrocyte membranes of three family members affected with aceruloplasminemia. All of them had elevated C26:1/C22:0 and C26:0/C22:0 ratios. These findings suggest that free radicals generated in persons with aceruloplasminemia may interrupt the peroxisomal beta-oxidation of fatty acids.