The relative accuracy of a semi-automated portable blood pressure (BP) measuring device, Terumo ES-H51, which is based on the auscultatory method, was assessed by comparison with simultaneous sphygmomanometric measurement on 48 patients, including hypertensive patients. During rest, mean differences and limits of agreement (mean +/- 2SD) were -1.3 mmHg, -7.9 and 5.3 mmHg, respectively for systolic BP, and -0.2 mmHg, -8.4 and 8.0 mmHg, respectively for diastolic BP. Although the differences between ES-H51 and sphygmomanometric measurement did not increase with repeated measurements or with changes of BP during the induction of anesthesia, blood pressure measurement by ES-H51 was sometimes impossible or erroneous during upper abdominal surgeries in 5 patients. We concluded that ES-H51 has satisfactory accuracy, and is suitable for blood pressure assessment during the peri-operative period but not during surgery.