Defective lipoprotein lipase (LpL) was found in the postheparin plasma (PHP) of a patient with severe hypertriglyceridaemia. The patient was a 14-year-old girl with a maximum plasma triglyceride (TG) level of 3600 mg d-1 who had been suffering from recurrent pancreatitis. The patient's LpL purified from the PHP by heparin-Sepharose and phenyl-Sepharose chromatographies hydrolysed tributryrin, but not triolein emulsified with Triton X-100 and phosphatidylcholine (PC), or in chylomicrons, whereas normal LpL hydrolysed these substrates. Moreover, unlike normal LpL, LpL from the patient did not associate with VLDL, as shown by Sepharose 4B column chromatography. The patient's LpL hydrolysed triolein emulsified with lysophospholipid at a normal rate in the presence of apolipoprotein CII. These findings suggest that this patient has LpL with a normal catalytic site for tributyrin but with a defect in lipid interface recognition resulting in loss of ability to recognize VLDL or chylomicrons, but not of triolein emulsified with lysophospholipid.