Serum lipoprotein pattern, apoproteins and two postheparin triglyceride lipases were analyzed in a patient with familial lipoprotein lipase (LPL) deficiency and her family. Serum of the patient showed extreme hyperchylomicronemia and her postheparin plasma LPL activity was distinctly decreased. None of heterozygotes had any type of hyperlipoproteinemia. The mother and brother of the patient had moderately decreased LPL activity. There were no consistent changes in hepatic triglyceride lipase (H-TGL) activity among heterozygotes. These results suggest that assay of LPL may be helpful for detection of heterozygotes in familial LPL deficiency.