Ninety-three patients presenting with acute asthma to the emergency department were studied to determine theophylline levels and their relationship to airway obstruction, history of prior medication use, and side effects of treatment. The mean pretreatment theophylline level was 6.4 micrograms/ml. Patients on long-acting preparations had significantly higher levels than those on short-acting medications (p less than 0.05). The mean post-treatment theophylline level was 16.7 micrograms/ml. Twenty-three patients had toxic levels post-treatment but none of these had a major adverse reaction. Twenty of these patients had been taking long-acting preparations. Fifty percent of the patients with symptoms of gastrointestinal toxicity had theophylline levels below 15 micrograms/ml. There was no correlation between the theophylline level or change in level and the degree of airway obstruction as measured by pulmonary function testing. Clinical findings are not reliable predictors of theophylline levels. Patients taking long-acting theophylline products should receive a lowered loading dose.