A new portable noninvasive recorder (4 X 6.5 X 14 cm in size, 390 g in weight) was developed for monitoring 24-hour blood pressure and its clinical applicability was investigated. Employing an ordinary-size cuff, this is the lightest and the most compact apparatus of its kind ever developed. It is powered by a rechargeable battery. The cuff is pressurized by a miniature, low-noise, rotary micropump. To eliminate noises resulting from body motion, two microphones are used to distinguish Korotkoff sounds. Systolic blood pressure (SBP), diastolic blood pressure (DBP), and heart rate (HR) are measured automatically at intervals of 1 to 60 min throughout 24 hours. These data can be stored as many as 600 times in the recorder's semiconductor memory. After measurement, mean values; standard deviations (SD); and trendograms of SBP, DBP, and HR are printed out by means of an appurtenant, miniature analyzer measuring 5 X 7.5 X 15 cm. A comparison of values obtained with this new instrument and the values obtained with a conventional auscultatory method showed average errors of -1.2 +/- 4.7 (SD)mmHg for SBP and -2.7 +/- 5.0 mmHg for DBP. The correlation coefficient (r) of values obtained by two methods was r = 0.99 for SBP and r = 0.96 for DBP (n = 185). In 38 male and 31 female normotensive subjects (average casual BP: male 126 +/- 11/76 +/- 7 mmHg; female 116 +/- 13/69 +/- 10 mmHg), average 24-hour BP values recorded by the new recorder were 115 +/- 7.5 (SBP)/70 +/- 6 (DBP)mmHg for the males and 106 +/- 6/63 +/- 5 mmHg for the females. The new recorder seems to be convenient, easy to operate, and clinically useful in ambulatory monitoring.