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To measure blood pressure accurately, by developing a new algorithm and an indirect method employing a non-elastic cuff with a photosensor.A non-elastic cuff was wrapped around the brachium. A reflecting plate (10 mm × 40 mm) was sited on the inner central part of the cuff on the arterial side. On the opposite side of the cuff, a photosensor consisting of light-emitting and light- receiving elements was positioned. Oscillation due to arterial pulsation was measured photologically during cuff inflation. Cessation of oscillation was taken as the systolic blood pressure (SBP) and, when the delay time between photo-oscillation and cuff-pressure oscillation upstrokes was at its minimum, the transition from rapid to more stable changes in photo-oscillation was taken as the diastolic blood pressure (DBP; the delta-algorithm). We compared the blood pressures measured directly in the brachial artery with those obtained by this method and by the conventional auscultatory method in 10 normotensive and 26 hypertensive subjects [12 women and 24 men, mean age 45 ± 16 years (mean ± SD), with mean brachial circumference 27 ± 2.6 cm]. The blood pressure of each subject was measured simultaneously by the direct and indirect methods five times.Errors (differences from direct blood pressure measurements) produced with this new method [SBP–0.8 ± 3.1 mmHg, (mean ± SD) DBP 0.6 ± 2.8 mmHg] were significantly smaller than errors obtained by use of the auscultatory method (SBP −7.4 ± 5.6 mmHg, DBP 3.0 ± 7.1 mmHg; P < 0.001, n = 180). The delta-algorithm was easily applicable to an automatic blood pressure measuring device.This new photo-oscillometric method was more accurate than the auscultatory method for measuring blood pressure. The delta-algorithm is logical and will be useful for accurate blood pressure measurement.