The purpose of this study was to assess the accuracy of the Omron HEM403C blood pressure (BP) device and to study whether this accuracy was affected by subject-related factors such as BP level and age. A comparison was made with a Hawksley random zero sphygmomanometer. BP was measured with both meters connected to one arm-cuff by means of a T-connector. To assess the level of agreement of the Omron HEM403C with the random zero meter, BP was measured in 212 subjects. Systolic and diastolic readings with the Omron HEM403C were on average 1 mmHg (s.d. 9, 95% CI 0.4-2.2) and 4 mm Hg (s.d. 8, 95% CI 3.6-5.1) respectively, higher than those with the random zero meter. The Omron HEM403C underestimated SBP compared with the random zero at higher SBP levels and overestimated SBP at lower SBP levels. The Omron meter overestimated the diastolic pressure at all DBP levels and the diastolic difference was not related to the DBP level. A multiple linear regression analysis demonstrated that the SBP level and not age was the factor that independently influenced the systolic difference between the meters. According to the criteria of the British Hypertension Society the Omron HEM403C is accurate enough to be recommended for those clinical and research purposes that are in need of automatic measurements.