Skin fibroblasts were cultured from young adult patients with Refsum's disease, an inherited metabolic disorder characterized by a deficiency in oxidation of phytanic acid and by increased serum and tissue concentrations of this fatty acid. These cultures were compared to cultures of normal fibroblasts in terms of the number and distribution of peroxisomes demonstrable cytochemically in preparations incubated for catalase activity. Refsum's fibroblasts were found to contain 1-10 peroxisome profiles per 100 micron 2 of cytoplasm; the controls contained 1-2 profiles per 100 micron 2. The peroxisomes in normal fibroblasts were found in all regions of the cytoplasm. In the Refsum's material they were relatively scarce in the perinuclear region, where many of the cells showed numerous large inclusions containing lipid-like material and myelin figures. Our findings indicate that in the adult form of Refsum's disease, which is the more thoroughly studied variety, peroxisomes in fibroblasts are not diminished in number. This contrasts with a recent report concerning a case of what is thought to be an infantile form of the disorder, in which no peroxisomes were detected in a liver biopsy. If phytanic acid accumulations in the adult form are a consequence of peroxisomal defects, the defects presumably are at the level of specific enzymatic deficiencies and do not involve a generalized absence of peroxisomes.