Although acute mountain sickness (AMS) has been studied for well over a century, a standard measure or index of the degree of illness for use in experimental research does not exist. This paper outlines a definition and procedures for an operational measurement of AMS using the Environmental Symptoms Questionnaire (ESQ). After 58 men completed over 650 ESQs during a stay of 1-3 weeks atop Pike's Peak (4300 m), factor analysis produced nine distinct symptom groups, with two factors representing AMS. The first factor contains symptoms indicative of cerebral hypoxia and is labeled AMS-C. The second reflects respiratory distress and is called AMS-R. Signal detection theory was used to establish a criterion score value for each factor. Standard deviation values were used to derive indices of sickness severity. Discussion is given to the possible relationships between the two types of AMS and the more serious conditions of cerebral and pulmonary edema.