Patients with type 1 hyperlipoproteinaemia present with relatively normal plasma levels of very-low-density lipoprotein (VLDL) together with marked hyperchylomicronaemia. Since the aetiological basis for the type 1 phenotype is a severe reduction in peripheral lipoprotein lipase (LPL) activity, the discrepancy between the two triglyceride-rich lipoprotein fractions is paradoxical. In order to account for these observations we have previously proposed (G.M.B) that the hepatic secretion of lipoproteins in these patients is characterized by the production of chylomicron-like particles rather than VLDL. The implications of this hypothesis were examined in 3 patients with type 1 hyperlipoproteinaemia. A high-carbohydrate diet led to the enhancement of hyperchylomicronaemia in 2 of the 3 subjects. The severity of the defect in peripheral LPL activity correlated with the tendency to hyperchylomicronaemia, and in vitro assay suggested that hepatic lipase was capable of hydrolysing chylomicron triglyceride when present at the high concentrations characteristic of type 1 hyperlipoproteinaemia. These results were compatible with the above hypothesis implicating the liver as a partial determinant of the type 1 phenotype.