BACKGROUND: The measurement of health outcomes has become a priority for assessing and containing health care costs. OBJECTIVE: To develop and fully validate a simple symptom scale assessing both asthma and rhinoconjunctivitis, two interdependent conditions. METHODS: A self-administered questionnaire of 31 items was tested in 102 patients with asthma and rhinoconjunctivitis between September 1992 and December 1994. The items were rated on a six-point Likert scale ranging from none to extremely severe and included commonly recorded symptoms of inflammation such as irritation, congestion, and discharge in the skin, eyes, nose, sinus, pharynx, and chest. RESULTS: Validation included (1) responsiveness: scores for 18 patients evaluated during an emergency room visit were statistically significantly different from those recorded after recovery and in a control group of 24 patients with asthma; (2) reliability: external consistency was 0.8 at 1 week, and internal consistency was 0.8 for individual organs and 0.7 for individual symptoms; (3) validity: a five-point scale better captured the distribution of values; irrelevant and redundant items were eliminated. The optimal questionnaire included 21 items. CONCLUSIONS: The symptom score was applicable, responsive, reliable, and valid. Used with existing validated tools such as treatment needs and quality of life assessment, it may provide a comprehensive picture of allergic airway disease for quality assurance or research purposes.