Plasma samples from several Zellweger patients were found to contain elevated phytanic acid levels. It was subsequently found that the level of phytanic acid in plasma from Zellweger patients depends upon the age of the patients at the time of sampling. In patients 17 weeks of age or younger, plasma phytanic acid levels were found to be normal, whereas in patients 40 weeks of age or older plasma phytanic acid levels were found to be elevated. The relationship between the age of the patients at sampling and the level of phytanic acid in the patients' plasma is probably the resultant of dietary intake of phytanic acid combined with a defective catabolism of this compound.