Peroxisomes play an essential role in mammalian cellular metabolism, particularly in oxidation fatty acid pathways. Serum very long-chain fatty acids (VLCFA), the main biochemical diagnostic parameters for peroxisomal disorders, were examined in 25 neurological patients with epilepsy on a ketogenic diet and 27 patients with liver dysfunction. The data show that patients on a ketogenic diet have increased levels of C22:0 and C24:0, but not C26:0, and normal C24:0/C22:0 and C26:0/C22:0. Patients with liver insufficiency showed a slightly elevated level of C26:0, a normal level of C24:0 and a decreased level of C22:0; thus in 21/27 the ratio of C24:0/C22:0 was increased and 15/27 the ratio of C26:0/C22:0 was increased.