The study developed a key piece of information needed for outpatient care of patients with asthma; a profile of patients at risk for severe, potentially life-threatening exacerbations of asthma. The experimental design was a prospective study of 800 patients with asthma drawn from a large, pre-paid health care plan. The key advantage of this population was that all the care occurred within the health plan. Characteristics identified included demographic factors, socioeconomic status; patient characteristics such as tobacco use, atopy, pattern of medication use, self-reporting of compliance/adherence, attitudes about asthma; historical assessment of indoor air quality; characteristics of asthma such as duration of asthma, lung function, and variation in asthma symptoms; and speciality of physician. The outcome measures were obtained over a three year period and included all episodes of hospital-based care including hospitalizations, emergency room visits, and urgency care clinic visits. Based on these data, models were developed which assigned relative risk to each of the independent variables. These models were validated in a subset of the population.