The activation of palmitate and trihydroxycoprostanic acid and the peroxisomal oxidation of palmitate, trihydroxycoprostanic acid and their CoA esters were measured in homogenates prepared from fresh liver tissue of patients undergoing hepatic surgery and from frozen postmortem liver specimens of controls, patients with Zellweger syndrome and a patient with pseudo-Zellweger syndrome, a deficiency of peroxisomal 3-oxoacyl-CoA thiolase. In contrast to the findings in control livers, peroxisomal beta-oxidation of palmitate and of palmitoyl-CoA was severely impaired, and oxidation of trihydroxycoprostanic acid and its CoA ester could not be detected in the livers of the patients affected by peroxisomal diseases. The finding in this paper, that the oxidation of trihydroxycoprostanoyl-CoA can be measured reliably in small amounts of human liver, will be of valuable help in the differential diagnosis and classification of peroxisomal disorders and will help to elucidate the exact nature of some of the defects present in these disorders.