Studies describing the effects on blood pressure control by hemodialysis(HD) or continuous amulatory peritoneal dialysis(CAPD) have yielded conflicting results with respect to 24 hour blood pressure control, diurnal variation and blood pressure loads. The aim of the study was to investigate the effect of beginning with HD or CAPD on blood pressure control, diurnal variation and blood pressure loads using ambulatory blood pressure monitoring(ABPM). Twen-ty-seven end-stage renal disease(ESRD) patients(12 on HD and 15 on CAPD) were enrolled into the study. Patients with cardiovascular diseases, erythro-poietin therapy, or severe edema were excluded. ABPM were performed two times before and after the initiation of dialysis. Mean duration of interval between pre- and post-dialysis ABPM were 17+/-4 days on HD and 13+/-3 days on CAPD. Daytime and nighttime were defined as the time from 6:00 AM to 10:00 PM and from 10:00 PM to 6:00 AM of the next day. Systolic and diastolic loads were defined as the percentage of the incidence of systolic and diastolic blood pressure over l% and 90mmHg. Dipper meaning the presence of normal diurnal difference were defined as the differences of daytime- nighttime mean arterial pressure more than 5mmHg. In HD patients, mean systolic and diastolic blood pressure and mean arterial pressure were significantly decreased after dialysis during 24 hour, day- time and nighttime. In CAPD patients, those were also significantly decreased after dialysis during 24 hour, daytime and nighttime(p<0.05). Diurnal differences were increased after CAPD(3.3+/-9.4 vs 5.4+/-6.8mmHg) but decreased after HD(4.3+/-6.2 vs 2.4+/-10.8mmHg) and the differences of diurnal difference between two groups were significantly different(+2.1+/-9.0 vs 1.9+/-8.4mmHg, p<0.05). Proportions of dipper among patients were increased from 16.7 to 66.7% in HD and from 33.3% to 60% in CAPD without statistical significance between two groups. Systolic and diastolic loads were significantly decreased after HD(from 75.0+/-38.0 to 37.5+/-43.8%, from 45.2+/-29.7 to 12.5+/-12.8%, respectively, p<0.05) and after CAPD(from 63.1+/-30,1 to 32.3+/-27.1%, from 43.4+/-36.2% to 12.2+/-16.9%, respectively, p<0.05). Systolic and diastolic loads of daytime and nighttime were significantly decreased after each dialysis modality except nighttime diastolic pressure load in HD. In conclusions, both HD and CAPD improve BP control in ESRD patients. CAPD has more benefit to control of diurnal variations in ESRD patients.