Forty-three patients with clinically and biochemically unequivocally defined type III hyperlipoproteinemia (HLP) were screened for the presence of the apolipoprotein (apo) B-100 arginine3500-->glutamine mutation. This receptor-binding defective apolipoprotein B variant is the cause of familial defective apo B-100 (FDB), an autosomal dominantly inherited disease, which leads to increased plasma cholesterol levels and premature atherosclerosis. Neither patient expressed FDB. It is concluded that the gene defect responsible for FDB is not involved in the pathogenesis of type III HLP.