Afibrinogenemia and hypobetalipoproteinemia in a kindred. A 3-year-old boy with minor bleeding problems had no plasma fibrinogen measured by both clottable assay and immuno-precipitation. Low normal fibrinogen levels were present in the mother and father. Markedly decreased plasma cholesterol and apolipoprotein B levels were found in the father, proband's brother, and the paternal side of the kindred. The proband and his mother had normal plasma total cholesterol and apolipoprotein B levels. These findings are compatible with autosomal dominant transmission of hypobetalipoproteinemia and autosomal recessive transmission of afibrinogenemia. Two members of the father's family had plasma cholesterol levels below the fifth percentile but elevated levels of fibrinogen (6.0 and 4.4 g/L). Both have symptomatic coronary heart disease. Finding coronary heart disease with very low cholesterol but elevated fibrinogen levels is consistent with fibrinogen levels being an independent risk factor for coronary heart disease. 