Hypertriglyceridemia is an important risk factor for atherosclerosis. In the fasting state, the triglyceride (TG) concentration is correlated significantly with the high-density lipoprotein-cholesterol (HDL-C) and apolipoprotein CIII (apoCIII) concentrations. A postprandial change is evident in TG, but negligible in HDL-C and apoCIII. We investigated whether the fasting TG concentration could be estimated from the postprandial HDL-C and apoCIII concentrations. We measured the TG, HDL-C, and apoCIII concentrations at seven points a day in 58 inpatients. Multiple regression analysis showed that the actual fasting TG concentration was strongly correlated with the TG concentration estimated from the fasting HDL-C and apoCIII concentrations (ln[TG](fasting)=0.0140[apoCIII](fasting)-0.724[HDL-C](fasting)-0.142, r=0.852, p<0.001). This equation was also fit to the fasting data from 163 outpatients (r=0.883, p<0.001). Although the TG concentration increased by up to 28.2%, the HDL-C and apoCIII concentrations changed little during the day. When we substituted the postprandial HDL-C and apoCIII concentrations for the respective fasting values in this equation, there were still strong positive correlations (r=0.794-0.840) between the actual and estimated fasting TG concentrations throughout the day. In conclusion, the fasting TG concentration can be estimated from the postprandial HDL-C and apoCIII concentrations.