Apolipoprotein (apo)E gene epsilon4 allele carrier status modulates the responses of lipoprotein metabolism to hormone-replacement therapy (HRT). We investigated the effect of long-term HRT on the progression of atherosclerosis in postmenopausal women with or without apoE epsilon4 allele. One hundred forty-one nonsmoking postmenopausal women, 45-71 yr old, were divided into 3 groups based on the use of HRT. The HRT-EVP group (n = 61) used sequential estradiol valerate (EV) plus progestin (P), the HRT-EV group used EV alone (n = 40), and a control group had no HRT. Of these 141 women, 93 participated in a 5-yr follow-up study in 1998. In addition to serum lipid concentration and apoE genotype, the atherosclerosis severity score of the abdominal aorta and carotid arteries was determined by sonography. In apoE4-negative subjects, the progression of atherosclerosis severity score was significantly faster in control than in the HRT groups (genotype-by-time interaction P = 0.0026); whereas in apoE4-positive subjects, there were no significant differences in atherosclerosis severity score progression between the control and HRT groups. The effects of HRT on atherosclerosis progression in subjects with no apo epsilon4-allele seems to be especially beneficial, compared with controls with same phenotype status but without HRT. These results may help us to understand, in more detail, the benefit and possible risk of HRT on atherosclerotic diseases.