Method of operating a non-invasive blood pressure, NIBP, monitor to measure the blood pressure of a subject, the NIBP monitor comprising a cuff, a pressure sensor for measuring the pressure in the cuff and for outputting a pressure signal representing the pressure in the cuff and a physiological parameter sensor, the method comprising obtaining a first measurement of pulse rate or heart rate for the subject during inflation of the cuff, the first measurement being obtained from the pressure signal; obtaining a second measurement of the pulse rate or heart rate for the subject during inflation of the cuff, the second measurement being obtained from the physiological parameter sensor; comparing the first measurement and the second measurement; and estimating the reliability of a blood pressure measurement obtained by the NIBP monitor during inflation of the cuff based on the result of the step of comparing.