We have conducted a prospective analysis of all reported critical incidents which occurred during the period from admission to the anaesthetic room until discharge from the operating theatre complex over a 1-year. The aim was to identify which monitors were most effective for initial recognition of those incidents. All patients were monitored in accordance with the recommendations of the procedures gave rise to 338 recorded critical incidents, of which 27 were deemed to have caused, or been likely to cause, patient harm. We have shown that a combination of arterial blood pressure measurement (noninvasive), ECG, pulse oximetry and clinical observation detected 90% of all reported critical incidents, and all the incidents causing patient harm. We have also shown that blood pressure monitoring by itself, and combination with ECG and pulse oximetry, detected proportionately more critical incidents resulting in patient harm (incidents resulting in harm remaining incidents, for each monitor) than clinical observation alone (p < 0.05) and all other remaining forms of monitoring together (p < 0.01).