The accuracy of a semi-automated ambulatory blood pressure monitoring device (the Instrumedix 'Personal Blood Pressure Transmitter') was assessed in 36 patients undergoing diagnostic cardiac catheterisation. Blood pressure readings measured by the device and by a standard manual sphygmomanometer were compared with simultaneous intra-aortic pressure measured directly via the catheter. No measurement size trends were identified for either of the two comparisons. Compared to direct intra-aortic pressure measurement, the manual sphygmomanometer gave lower systolic (p less than 0.05), and higher diastolic readings (p less than 0.01), while the semi-automated device gave slightly higher readings for both systolic (not significant: p greater than 0.05) and diastolic (p greater than 0.05) pressure. In this assessment the semi-automated device provided at least as accurate a reflection of intra-aortic pressure as did standard manual sphygmomanometry. This device is an accurate tool for the ambulatory assessment of blood pressure where adequacy of treatment or correctness of the diagnosis is in question.