The validity and clinical performance of the ES-H531, a newly developed ambulatory blood pressure (BP) monitoring device, were evaluated. The validity was assessed by simultaneous comparisons (total of 223 measurements) with the measurements made by two experienced observers using standard mercury column sphygmomanometers in 121 subjects. The clinical performance was evaluated through the number of measurement failures by observation of the cuff equipped site, and tolerability was assessed by administering a questionnaire to 30 of the subjects following completion of 24-h BP monitoring. The differences in systolic and in diastolic BP measured by the standard auscultation method and that measured with the ES-H531 by the Korotkoff method (K method) was 0.9+/-3.7 mm Hg (mean+/-standard deviation) and 0.1+/-3.2 mm Hg, respectively. These values satisfy the accuracy criteria of the American Association of Medical Instruments (AAMI) and accord with the grade of A in the accuracy criteria of the British Hypertension Society (BHS). The mean difference in systolic and in diastolic BP as assessed by the standard auscultation method and with the ES-H531 by the oscillometric method (O method) was 1.1+/-4.6 mm Hg and 2.7+/-5.4 mm Hg, respectively. This accords with grade B in the BHS accuracy criteria. Failures or errors in measurement occurred in 4.4% of the readings. A few subjects experienced discomfort because of noise or pain during measurements. The ES-H 531, a new compact and light weight ambulatory BP monitoring device, offers acceptable validity and sufficient comfort for clinical use.