Objective:To assess the Accutracker II (version 30/23) ambulatory blood pressure monitor by nationally agreed protocols in order to resolve previous conflicting assessments, and to examine the feasibility of combining these protocols into one study.Design:The protocols of the Association for the Advancement of Medical Instrumentation (AAMI) and the British Hypertension Society (BHS) were used simultaneously.Subjects:Five normotensive subjects were used to assess interobserver variation; 30 subjects took part in the field evaluation and 85 in the laboratory evaluation. The latter subjects were selected to cover a wide range of blood pressures and differences in arm circumference.Outcome measurements:Classification of device accuracy according to the criteria of the BHS and AAMI protocols.Results:Among 255 observations the mean ±SD difference between blood pressure measurements by the Accutracker II and the observers was −2.2±0.8/ −3.5±0.9mmHg. There was a small but statistically significant difference between one device and the other two, but all three fulfilled the BHS criteria of 95% of measurements falling within 3 mmHg before and after use. In 30 24-h recordings >80% of the programmed inflations produced valid recordings. Editing criteria accounted for the majority (55%) of invalid readings; weak Korotkoff sounds, imperfect electrocardiogram signals or movement artefact accounted for the remainder.Conclusions:Version 30/23 of the Accutracker II fulfilled the AAMI criteria; using the BHS system, it was graded A for systolic and C for diastolic blood pressure. Although both protocols were readily combined into one study, they do not assess exactly the same aspects of blood pressure measurement. Previous conflicting evaluations could be due to differences in applying the protocols or may result from modifications in production models. Posture may affect the rating given by the BHS protocol. Until there is general international agreement on the method used to validate ambulatory blood pressure monitors, simultaneous use of both the BHS and AAMI protocols is recommended. In future assessments or investigational use of any similar instruments the model used should be described precisely.