The inheritance of broad-β disease (as specified by a type III lipoprotein pattern) has remained an enigma. Previous reports have variously implicated a single gene (autosomal dominant mode), a double dose of a single gene (autosomal recessive), two separate genes (mixed heterozygosity), or multiple genes (polygenic inheritance). The present study of a single, large kindred of 108 members spanning 4 generations provides evidence for an autosomal dominant mode, since at least 1 member of the first generation pair was normal, at least 5 of their 9 children had type III patterns, and at least 2 of these (whose spouses were normal) transmitted this pattern to their offspring. The findings also suggest that in this kindred the common occurrence of hypertriglyceridemia (in a type IV pattern) may represent either a variable phenotypic expression of the gene for broad-β disease or the coexistence of a second, independent genetic lipid disorder.