We treated 14 patients who had an initial serum theophylline concentration greater than 30 micrograms/ml (48.3 +/- 19.4 micrograms/ml) and symptoms of theophylline toxicity with oral activated charcoal (OAC). Thirty-gram doses of OAC were administered approximately every two hours for two to four doses. Ten patients tolerated OAC and demonstrated a reduction in theophylline half-life to 5.6 +/- 2.5 hours with resolution of symptoms. Three of these ten patients were treated in the emergency department and discharged, making hospitalization unnecessary. The four patients with the highest initial theophylline concentrations (76.6 +/- 17.7 micrograms/ml) vomited all doses of OAC. Three of these four patients were treated with charcoal hemoperfusion with a reduction in the half-life to 5.2 +/- 1.0 hours. These data support the use of OAC as the primary therapeutic modality in the management of patients with theophylline toxicity. Patients with very high theophylline concentrations (greater than 50 micrograms/ml), however, usually vomit the OAC and may require charcoal hemoperfusion.