Cardiac cycle phase uncertainty causes a small error in indirect estimation of arterial blood pressure by sphygmomanometry. A simple analysis yields a statistical description of the error and a rule for its reduction: correct bias by adding to the systolic estimate and subtracting from the diastolic estimate one-half of the per-cardiac-cycle cuff deflation decrement. If several measurements are taken, the least-square estimate is approximately the arithmetic mean of the greatest and least of the bias corrected estimates.