Three cases of severe slow-release theophylline toxicity (plasma theophylline levels above 30 mg/l) in patients with severe chronic pulmonary disease are reported. Seizures were noted in all patients, and one had tachyarrhythmia. Two of the patients died. Several factors were present which may have impaired theophylline clearance, leading to toxic plasma levels. The mortality rate of theophylline intoxication being high, the initial doses should be sufficiently low to minimize the risk of adverse affects, and the best way to prevent theophylline toxicity is to monitor plasma concentrations. A management of patients with theophylline overdosage is suggested: oral administration of activated charcoal is the primary therapeutic measure; haemoperfusion or haemodialysis should be considered only in patients in whom conservative measures have failed.