After a century of clinical use, the mercury sphygmomanometric technique of blood pressure (BP) measurement is under threat. There are three reasons for this: mercury is likely to be banned from hospital use because of the danger of toxicity, accurate automated devices are now available to replace the mercury sphygmomanometer and with the advent of 24 h ambulatory BP measurement into clinical practice, more reliance is being placed on BP behaviour rather than on casual measurements. The passing of the mercury sphygmomanometer from clinical practice raises two issues that merit deliberation. Without the mercury standard with which to compare measurements generated by algorithmic interpretation of BP, the clinician is dependent on the consistency and accuracy of such algorithms. If the millimetre of mercury is no longer the unit of measurement for BP, it will soon be replaced by the kilopascal thereby introducing, at least temporarily, the potential for uncertainty in clinical judgment.