Asthma management is a complex process requiring the active, significant, and consistent attention of physicians and patients. Physician knowledge regarding asthma care was evaluated in this study as part of a comprehensive health improvement strategy to improve the quality of asthma care in health maintenance organizations (HMOs). Sixty-eight randomly selected physicians who treat asthma patients were interviewed on basic clinical knowledge and specific applications of knowledge in four domains: assessment/diagnosis, pharmacological therapy, asthma triggers, and patient management/education (72% participation). The standardized interview, based on 1997 NIH/NHLBI Expert Panel Report 2: Guidelines for the Diagnosis and Management of Asthma, was administered by a pulmonologist. Each domain contributed 25 points to the overall score of 100 points. Physicians were trained in pediatrics (46%), internal medicine (28%), and family practice (22%). Mean overall asthma clinical knowledge score was 46 6 9. Mean domain-specific scores were also low ranging from 9 6 3 for asthma triggers to 13 6 3 for assessment/diagnosis. Physicians exhibited clear deficiencies in assessing asthma severity based on symptoms and pulmonary function. Limitations were also observed in knowledge of pharmacotherapy, asthma triggers and control measures, and patient education/management. Physicians' knowledge of NIH treatment guidelines was limited. Particularly important was the under assessment of asthma severity. Effective patient treatment and management depends upon accurate classification of asthma severity. To reduce asthma morbidity and mortality, education efforts should emphasize improving physicians' severity assessment through better ascertainment of asthma-related symptoms and increased use of objective measures of lung function. (Pediatr Asthma Allergy Immunol 2000;14[4]:277-285.) 277 1 Previously published as an abstract. Reference: Ricci J, Stewart W, Geringer W, Murphy S. Physicians' knowledge of NIH asthma treatment guidelines. Am J Resp Care Crit Care Med 1999; 159(3):A240.