To detect genetic mutants of apo A-I, the major structural protein of human HDL, we screened 530 unrelated Austrian probands (168 children, 362 adults). An apo A-I mutant characterized by an exchange of the acidic amino acid Glu in position 198 with the basic amino acid Lys was identified in the serum of the mother of a hyperlipoproteinemic girl. So far only two patients with this mutant, referred to as apo A-I (Glu 198----Lys) have been described. We detected six new patients (two children and four adults) with apo A-I (Glu 198----Lys) among 20 members in three generations of the affected family. An autosomal codominant inheritance of the apolipoprotein variant could be established. All affected individuals were heterozygous for the mutant. Among the six new subjects with apo A-I (Glu 198----Lys) two children and one adult presented with high-density lipoprotein (HDL) cholesterol concentrations below the fifth percentile for age and sex and with low serum apo A-I and A-II. Although there was no consistent relationship of the mutant with low serum HDL in this family, a moderate effect of apo A-I (Glu 198----Lys) on HDL levels cannot be ruled out. Hyperlipoproteinemia of types IIa, IIb, and IV was observed in eight of the 20 family members studied, but did not cosegregate with the mutant apo A-I. There was no association of apo A-I (Glu 198----Lys) with premature clinical manifestations of atherosclerosis. The mutation occurred in a part of the apo A-I molecule, which is thought to be involved in lipid binding.(ABSTRACT TRUNCATED AT 250 WORDS)