It has recently been suggested that a substitution of glutamine for arginine at residue 3500 of apolipoprotein (apo) B-100 causes familial defective apo B-100 (FDB), an autosomal, dominantly inherited disorder, which leads to increased serum cholesterol levels. From a sample of 243 patients from Munich with type IIa hyperlipoproteinemia (HL), we have identified eight individuals with the apo B-100 arginine(3500)----glutamine mutation. In a group of 57 subjects with defective low density lipoprotein receptor (LDLR), no mutant apo B alleles were detected. The frequency of FDB in patients with type IIa HL was estimated to be 3%. In the kindreds of three of the probands, 10 additional carriers of the apo B mutation were identified. Clinical and biochemical data reveal a striking similarity between patients with FDB and those with a defect in the LDLR gene. Our data support previous findings that FDB is a serious disorder causing premature atherosclerosis.