The objective of this study was to determine the accuracy of the Microlife BP A100 blood pressure measuring device tested according to the requirements of the International Protocol of the European Society of Hypertension. The BP A100 model is designed to provide accurate blood pressure measurements over a wide range of arm circumferences using a single cuff with standard-size bladder. Device evaluation was performed on 33 patients with a mean +/- SD age of 59 +/- 19 years (range 30-89 years). Their systolic blood pressure (SBP) was 143 +/- 23 mmHg (range 108-180 mmHg), diastolic blood pressure (DBP) was 87 +/- 16 mmHg (range 60-108 mmHg), and arm circumference was 31 +/- 4 cm (range 22-42 cm). In nine patients, the arm circumference was at least 33 cm. Blood pressure measurements were performed in the sitting position. The BP A100 passed all three phases of the European Society of Hypertension protocol for SBP and DBP. Mean blood pressure differences between device and observer were -2.9 +/- 4.9 mmHg for SBP and -2.6 +/- 4.6 mmHg for DBP. Similar device-observer differences were observed in the patients divided into two subgroups according to whether their arm circumference was above or below the median in the group. In conclusion, the present results indicate that the Microlife BP A100 monitor coupled with a single cuff with standard size bladder provides accurate blood pressure readings over a wide range of arm circumferences.