A patient with classic Zellweger syndrome was treated with docosahexaenoic acid ethyl ester (DHA-EE) for three months. Five other patients with Zellweger variants (four of them less than one-year-old and a five-year-old) were treated with DHA-EE until normalization of the DHA levels in erythrocytes. When arachidonic acid (AA) concentration decreased, AA was added to the diet. Thereafter, a combined treatment with DHA plus AA followed, in a variable proportion that allowed the high levels of DHA in erythrocytes to be maintained. In the patient with Zellweger syndrome, DHA therapy produced an increase in plasmalogen and a decrease in 26:0 and 26:1. No clear clinical improvement could be detected in this patient during the short period of treatment with DHA-EE. The most consistent clinical effect produced by DHA therapy in the other patients with disorders of peroxisomal biogenesis was visual improvement, even in those patients that were virtually blind before the treatment. In general, the developmental curve began to accelerate. The infants became more alert, acquired better visual and social contact and muscular tone improved, with the beginning of good head control. The liver tests tended to normalize and some patients showed a reduction of hepatomegaly. All these favorable changes occurred when the patients were taking the DHA-EE alone. In some of the patients, muscular tone seemed to improve further after introducing AA supplements. From the biochemical point of view, the plasmalogen levels increased in most cases in erythrocytes, and the two ratios 26:0/22:0 and 26:1/22:0 decreased in plasma. In some patients there was a tendency for 26:1 to increase in plasma and for 18:0 plasmalogen to decrease in erythrocytes when AA was introduced in the diet. The significance of these findings remains to be elucidated, but they stress the importance of strict monitoring and control of the polyunsaturated fatty acids status during DHA therapy.