Although anxiety has both dispositional and situational determinants, little is known about how individuals’ anxiety-related sensitivities and their expectations about stressful events combine to determine anxiety. This research used Information Integration Theory and Functional Measurement to assess how participants’ anxiety sensitivity and event expectancy are cognitively integrated to determine their anxiety about physical pain. Two studies were conducted—one with university students and one with anxiety clinic patients—in which participants were presented with multiple scenarios of a physically painful event, each representing a different degree of event probability, from which subjective expectancies were derived. Independent variables included anxiety sensitivity (low, moderate, high) and event expectancy (low, medium, high, no probability information). Participants were asked to indicate their anxiety (dependent measure) in each expectancy condition in this 3 X 4 mixed, quasi-experimental design. The results of both studies strongly suggest that anxiety sensitivity and event expectancy are integrated additively to produce somatic anxiety. Additional results and their implications for the treatment of anxiety-related disorders are also discussed.