We investigated the possible therapeutic effect of decreasing plasma levels of very-long-chain fatty acids (C26:0) with a synthetic oil containing trioleate and trielucate (Lorenzo's oil) as well as increasing docosahexaenoic acid (DHA) in red blood cells (RBC) with DHA ethyl ester in four patients with Zellweger syndrome. We investigated serial changes of plasma C26:0 levels and DHA levels in RBC membranes by gas-liquid chromatography/mass spectrometry (GC/MS). After death, the fatty acid composition of each patient's cerebrum and liver was studied. Dietary administration of Lorenzo's oil diminished plasma C26:0 levels. Earlier administration of Lorenzo's oil was more effective and the response did not depend on the duration of administration. DHA was incorporated into RBC membrane lipids when administrated orally, and its level increased for several months. The final DHA level was correlated with the duration of administration and was not related to the timing of initiation of treatment. DHA levels in the brains and livers of treated patients were higher than in untreated patients. Early initiation of Lorenzo's oil and the long-term administration of DHA may be useful for patients with Zellweger syndrome.