OBJECTIVE: This study estimated the direct, indirect, and total costs of asthma and evaluated the impact of symptom severity, disease control, and the National Asthma Education and Prevention Program treatment guidelines on cost of asthma care using patient survey data. METHODS: Data were obtained from a 1993 US household survey to determine the costs of asthma (N=698). Costs were developed by assigning unit costs to reported medical utilization and unpaid resource usage and were reported in 1994 US dollars. A multivariate regression model was developed to determine factors associated with higher asthma costs. RESULTS: The average annual total cost of asthma care was US$966 per person, ranging from US$47 (controlled symptoms) to US$7,030 (uncontrolled symptoms). Costs for uncontrolled patients were due mainly to costs of hospitalization and indirect costs of missed days from work. Based on the reported incidence of asthma in the US, the cost of asthma care is estimated at US$9.6 billion. Significant predictors of higher direct, indirect, and total costs of asthma include symptom severity (p<0.001) and following pharmacotherapy guidelines (p<0.01). Younger age is associated with higher costs in the indirect and total cost models (p<0.01 and p<0.05). Smoking is associated with higher indirect costs (p<0.05). Lower total and direct costs are associated with patients who self-pay for health care (p<0.05). Lower indirect costs are associated with patients who report their asthma is controlled (p<0.05). CONCLUSION: Symptom severity is associated with higher asthma care costs. Our findings suggest that either people not meeting guidelines also have zero medical expenses or adherence to the treatment guidelines actually increases costs. A prospective study is necessary to determine the impact of guidelines.