Three hundred ten aneroid and 25 mercury sphygmomanometers in hospital use were calibrated against a new mercury manometer standard which properly recorded zero. A total of 68.4% (212 of 310) of the aneroid instruments and all of the mercury instruments were accurate within ± 3 mm Hg tolerance (average of four readings). An average deviation from standard of greater than ± 7 mm Hg was shown by 13.2% (41 of 310) of the aneroid instruments. Significant variation among hospitals in the proportion of accurate aneroid manometers appeared to be related to maintenance procedures. Aneroid sphygmomanometers must be calibrated periodically against a mercury manometer standard with a simple Y-tube connection. Only with careful observation, calibration, and maintenance should aneroid instruments be used in emergency care situations or in scientific studies concerned with indirect blood pressure.