A mild variant of Zellweger (cerebro-hepato-renal) syndrome was diagnosed in male and female siblings aged 7 and 2 years. They had mild facial dysmorphia, moderate psychomotor retardation, tapetoretinal degeneration, sensorineural deafness and hepatomegaly. Ultrastructural examination of a liver biopsy in the younger patient revealed the absence of recognizable peroxisomes. In both patients plasma levels of pipecolic acid, phytanic acid, trihydroxycoprostanoic acid and dihydroxycoprostanoic acid were elevated. The very long chain fatty acid C26:0 and the C26:0/C22:0 fatty acid ratio were elevated in plasma, but less than in classical Zellweger syndrome. In cultured fibroblasts, deficient acyl-CoA:dihydroxyacetone phosphate acyltransferase and increased concentrations of C26:0 as well as C26:1 very long chain fatty acids were found within the ranges previously established for patients with classical Zellweger syndrome. Particle-bound catalase was absent in fibroblasts. Despite the relatively mild clinical expression the biochemical abnormalities found in these patients are the result of a general peroxisomal dysfunction similar to the changes in classical Zellweger syndrome.