A case of primary type III hyperlipoproteinemia as defined by Fredrickson and Lees' system for phenotyping hyperlipidemias is presented. The patient exhibited a low plasma Thyroxine Binding Globulin (TBG) of unknown origin. These findings, as far as we know, have not been described previously and suggest the possibility that the defect in the lipoprotein synthesis is not the only metabolic defect in type III hyperlipoproteinemia. The patient further exhibited the occurrence in small amounts of non-identified, probably long-chained fatty acids in the plasma phospholipids, not seen in our normal subjects. The third abnormality was a pathologic plasma-insulin response to peroral glucose tolerance test. Family investigations gave no information regarding other cases of this type of hyperlipidemia among the relatives. A genetic relation to hyper-pre-beta-lipoproteinemia is possible. © 1969.