Takeda Medical (A & D) TM 2420 is an automatic ambulatory blood pressure monitoring system employing the auscultatory technique. The device was used under stable conditions and compared to readings from the Hawksley random-zero sphygmomanometer using a double headset stethoscope and a Y-connection. We tested 85 subjects (aged 13-89 years, systolic blood pressure 85-212 mmHg, diastolic blood pressure 40-116 mmHg) and found a difference amounting to 1.6 +/- 6.7 mmHg (mean +/- SD) for systolic and 2.1 +/- 4.5 mmHg for diastolic readings (Hawksley-TM 2420). In 62 subjects a comparison with simultaneous measurement on the opposite arm with the Hawksley manometer showed similar results. When comparing intra-arterial readings from 10 subjects, a difference (intra-arterial-TM 2420) of -1.9 +/- 12.1 mmHg was found for systolic pressures, while the diastolic difference was -10.7 +/- 8.7 mmHg. Twenty-four hour monitoring was performed on 80 subjects; 70 of these yielded usable tracings. The proportion of successful recordings was acceptable, but the device was not suitable for bicycle stress testing. The quality of the accessories provided with the equipment could be improved, but in spite of this the monitoring system was found to be recommendable for clinical use.