An episode of supraventricular tachycardia occurred in an infant born at 31 weeks' gestation after three days of therapy with theophylline 3.5 mg/kg every six hours. The infant had a plasma theophylline half-life of 24.7 hours and a low plasma theophylline clearance of 12.7 ml/kg/hour. The plasma theophylline concentration at the time of the arrhythmia was about 42 mg/l. The extent of drug accumulation with these kinetic characteristics is emphasized. Serious adverse effects due to theophylline may occur with few signs of symptoms of impending toxicity. Plasma theophylline concentration measurements are valuable in avoiding such toxicity.