Despite significant improvements in the safety of anesthetic equipment in recent years, disconnection in the patient breathing circuit remains one of the most common types of preventable anesthetic mishaps. Many currently available breathing-circuit monitors cannot reliably detect several hazardous conditions in the breathing circuit. An improved breathing circuit monitor has been developed that reliably detects realistic simulations of many hazardous conditions. The software algorithm in this monitor stores the normal pressure waveform in the patient circuit, evaluates the ongoing waveform in terms of sets of absolute and relative criteria, and warns the operator when such criteria are not satisfied. A comprehensive technical and clinical evaluation is underway.