Oral activated charcoal was administered to 4 patients with theophylline toxicity, who had gastrointestinal and neurologic symptoms as well as cardiac arrhythmias. Administration of oral charcoal significantly reduced the theophylline half-life in the serum of each patient. The charcoal was well tolerated and symptoms of toxicity rapidly resolved. We conclude that administration of oral charcoal rapidly reduces the serum theophylline concentration and should be used as the initial treatment in patients with theophylline toxicity.