High density lipoprotein-cholesterol (HDL-C) levels are inversely related to the incidence of coronary artery disease. We studied the influence of a G (-75) - > A transition in the promoter of the apolipoprotein (apo) A-I gene, a major protein component of HDL, on serum HDL-C levels in hyperlipidemic subjects. Seventy-three hyperlipidemic subjects with serum levels of high HDL-C (HDL-C 70 mg/dl, Group H) were compared with hyperlipidemic subjects with levels of HDL-C between 40 and 70 mg/dl (Group N) and those with HDL-C < 40 mg/dl (Group L). Group H showed a higher incidence (45.2%) of low plasma cholesteryl ester transfer protein (CETP) activity than Groups N (9.1%) and L (5.3%) (p < 0.001). Group H had a higher incidence of the G (-75) - > A transition (0.275) than Groups N (0.117, p < 0.05) and L (0.056, p < 0.01), among subjects with normal CETP activities. The HDL-C levels in subjects with the transition (84+16 mg/dl) were higher than those in subjects without the transition (56+12 mg/dl) (p < 0.05). These data suggest that a G (-75) - > A transition of the apo A-I gene promoter, in addition to the common mutation of CETP gene, contributes to high HDL-C levels among hyperlipidemic patients in Japan.