Male, identical twins presented with hypotonia, hypoglycaemia, dysmorphic facies, feeding problems, discoloured stools, hepatomegaly, and nephrolithiasis. Elevated blood levels of very long-chain fatty acids and bile acids suggested a peroxisomal disorder. Plasmalogen biosynthesis in cultured fibroblasts was reduced. Morphologically distinct peroxisomes were undetectable in liver. Twin 1 suffered from nephrocalcinosis and severe infection, and died at 18 months of age. Twin 2 was blind and physically severely retarded with epilepsy, but survived up to the age of 5 years. Studies of the fatty acid composition of serum lipids showed barely detectable values of eicosapentaenoic (EPA) and docosahexaenoic acid (DHA). During long-term treatment with these n-3 fatty acids, started at age 10 months, the fatty acid profile of the serum lipids was improved or normalized. Since n-3 fatty acids are essential elements in normal development, notably of the nervous system, we suggest that treatment with EPA and DHA should be started as early as possible in general peroxisomal disorders.