The infantile and classical forms of Refsum's disease are generally considered to belong to the newly recognized group of peroxisomal disorders. In this study we carried out a detailed investigation into different peroxisomal functions in classical Refsum's disease by analyses of plasma (very long chain fatty acids, di- and trihydroxycoprostanoic acid and pipecolic acid) and cultured skin fibroblasts from the patients (de novo plasmalogen biosynthesis, very long chain fatty acid oxidation and amount of particle-bound catalase). The results obtained indicate that, except for a deficient phytanic acid oxidation, peroxisomal functions were found to be normal in classical Refsum's disease in contrast with the findings in infantile Refsum's disease, in which there is a general impairment of peroxisomal functions. Based on these results it is concluded that peroxisomal biogenesis is normal in classical (but not in infantile) Refsum's disease and that the classical and infantile form of Refsum's disease hence represent distinct entities. Since available evidence suggests that phytanic acid is oxidized in mitochondria rather than in peroxisomes, at least in rat liver, it remains to be established whether classical Refsum's disease is a peroxisomal disorder or not.