The clinical and biochemical features of the heterozygotes of lipoprotein lipase deficiency have not been fully understood, because the diagnosis of the heterozygote state has not been established. Babirak et al. recently reported that combined measurements of postheparin plasma LPL activity and of LPL mass allowed identification of the heterozygote state for LPL deficiency, some of which were found to have mild lipoprotein abnormalities.<br>Here we described a case of the heterozygote who exhibited chylomicronemia and was diagnosed by measurements of LPL activity and mass.<br>The patient was a male infant, who had multiple anomalies and growth retardation. Chylomicronemia was observed at six months of age. Measurements of serum lipids and apolipoprotein showed extreme hypertriglyceridemia (maximally 8, 280mg/dl at eleven months of age) and high apo C-II level. These clinical course and laboratory data were compatible with LPL deficiency, but postheparin LPL activity measured with artificial substrate and LPL mass measured in an enzymelinked immunoassay (ETA) were 58% and 27% of their normal values, respectively. This data accorded with the heterozygote state of LPL deficiency.<br>Measurements of LPL activity and mass also revealed that his mother and two sisters were heterozygotes, who were grossly normolipidemic. Their LPL activity was lower than the proband, but their LPL mass was greater than his. So in this family, LPL mass seemed to correlate better with LPL activity in vivo.<br>In any case, his LPL activity and mass were too great to cause chylomicronemia. Thus, it may be reasonable to speculate that other factors had temporally inhibited his LPL activity, causing transient chylomicronemia. We were not able to identify these factors, but production of LPL inhibitor as a consequence of bacterial infection may partly explain why he exhibited chylomicronemia.<br>This case showed that heterozygotes of LPL deficiency present severe chylomicronemia under certain conditions. It also suggests that their lipoprotein abnormalies have yet to be determined.