A new instrument capable of recording systolic and diastolic blood pressure automatically and repetitively is described, and its accuracy is shown to be equal to that obtained by a carefully trained observer among 53 patients with different levels of blood pressure. The instrument (BosoMat) is also shown to be more accurate than two others available, namely, the Arteriosonde Model 1213 and the SphygmoStat Model B-300. The standard deviation among three consecutive blood pressures on the same patient as determined by the new instrument or by the nurse using traditional methods is shown to be approximately 4 mm. Hg.