Both in clinical practice and medical research, blood pressure is still largely measured by auscultation using a mercury sphygmomanometer. Blood pressure is the most important predictor of life expectancy. Treatment of high blood pressure reduces strokes, heart attack and heart failure. Accurate measurement is therefore essential. At a large London teaching hospital, just under 500 mercury sphygmomanometers and their associated cuffs were examined. More than half had serious problems that would have rendered them inaccurate in measuring blood pressure. At the same time, assessment of the technical knowledge needed to measure blood pressure by the ausculatory technique was also carried out amongst medical and nursing staff. This showed a considerable level of ignorance. These results inevitably lead to inaccurate measurement of blood pressure with serious consequences. In addition mercury is a non-degradable pollutant, eventually accumulating on the sea bed. The use of mercury in sphygmomanometers is already in the process of being eliminated in Scandinavia and Holland and other countries are likely to follow. Our results suggest that mercury sphygmomanometers are not adequately maintained and require expertise that is not available for accurate measurement of blood pressure. Their use should be dispensed with on these grounds before a ban for other and, perhaps less justifiable reasons. Validated automatic devices, which are less liable to measurement and observer error should be used instead. At the same time a concerted effort is needed to instruct health care professionals on the importance of more accurate measurement of blood pressure. Journal of Human Hypertension (2000) 14, 31-36.