BACKGROUND: Decisions about how to treat patients with acute exacerbations of obstructive airways disease-chronic obstructive pulmonary disease (COPD), asthma or mixed diagnoses-often require an understanding of prognosis. This depends on the severity of the acute deterioration and the patient's functional reserve. There are currently no validated disease-specific scores that measure the severity of the acute exacerbation. OBJECTIVE: To develop an acute physiology score for exacerbations of obstructive airways disease. DESIGN: Secondary analysis of a high-quality clinical database, the Case Mix Programme Database. SETTING: One hundred and sixty-eight adult, general critical care units in England, Wales and Northern Ireland. RESULTS: A total of 8527 patients with obstructive airways disease were identified with a mean (SD) age of 65.9 (9.7) years and hospital mortality of 35.5%. The COPD and Asthma Physiology Score (CAPS) was developed using logistic regression. The CAPS included eight variables: heart rate, mean arterial blood pressure, pH, sodium, urea, creatinine, albumin and white blood cell count. The score had fair discrimination with an area under the receiver operating characteristic curve of 0.718. This performance was reproduced in a further validation dataset of 7957 patients. The discrimination of the CAPS in these validation data exceeded that of the acute physiology scores from APACHE II and III and the physiological components of SAPS II. CONCLUSION: The CAPS can be used to estimate the prognostic impact of physiological derangements accompanying an acute exacerbation of obstructive airways disease and has the potential for even greater predictive performance when combined with measures of a patient's functional reserve.