In 748 probands and 3,283 first-degree relatives from the Collaborative Lipid Research Clinics (LRC) Family Study, our specific aim was to examine the degree to which low (bottom decile) high density lipoprotein cholesterol (HDCC, hypoalpha) and high (top decile) triglyceride (TG, hyperTG) levels occur conjointly (CT) and the extent to which these characteristics were shared within families. To control for family size and permit a comparison with the proband percentages, mean familial percentages of HDCC/TG abnormalities were calculated. Concurrent low HDCC and high TG levels were present in 2.7% of the probands, a value that was enriched to 12.7% (p=0.003) of their associated first-degree relatives. If the proband had a low HDCC value, 7.7% (p=0.013) of relatives had CT. Familial (proband and at least one first-degree family member share the same lipoproteinllipid phenotype) hypoalpha was observed in 2.4% of families while familial hyperTG was observed in 4.1%. Familial CT was seen in approximately 0.7%. If the proband had CT, 80% of their families had at least one other first-degree member with an HDGC/TG abnormality, whereas the corresponding percentage for families associated with probands with only hypoalpha was 64% and for those with hyperTG alone, 54%. A broadly shared environmental factor cannot easily explain the familial association of hypoalpha, hyperTG, and CT. In probands with low HDL-C values alone or the conjoint low-HDCCIhigh-TG trait, family screening is extremely valuable because low HDGCIhigh TG is enriched in the respective family members, a conjoined trait closely associated with increased coronary heart disease risk.