Pulmonary embolism in a human patient can be detected or confirmed by monitoring the level of endogenous NO in expired air, comparing the measured level with baseline values previously obtained for the same patient or baseline values representing a healthy population. The method can be used alone, or as a supplemental or adjunct step in traditional methods for diagnosing pulmonary embolism. Analysis of the obtained NO value, with consideration of the flow and resistance, can be used to indicate the severity of the pulmonary embolism.