Several parameters of lipoprotein metabolism were examined in 38 men with primary hypertriglyceridemia (phenotype IV). Family investigation showed that 17 men had familial combined hyperlipidemia (FCH), seven had familial hypertriglyceridemia (FHT), and 14 had unclassified hypertriglyceridemia (UNC). In all three groups, plasma high density lipoprotein (HDL) cholesterol and the concentrations of apolipoprotein A-I and A-II were decreased, and apolipoprotein B was increased, each to the same extent. These results are compatible with an increased risk of cardiovascular disease in both FCH and FHT patients. The mean concentration of LDL cholesterol and the ratio of LDL to HDL cholesterol were significantly higher in FCH subjects, which could explain their increased risk. Postheparin lipoprotein lipase and hepatic lipase were the same in both groups. Determination of apolipoprotein C composition, which may modulate lipoprotein lipase activity, did not reveal any abnormalities in the different groups. In both FCH and FHT, the mean turnover rate of plasma triglycerides was almost twice normal, indicating that overproduction of plasma triglyceride plays an important role in both disorders. However, there was an overlap with normal controls, indicating impaired triglyceride removal in some subjects. The underlying mechanism of hypertriglyceridemia in FCH and FHT therefore seems to be heterogeneous.