The aim of this study is to assess in a prospective fashion the causes and the frequency of overdosage with Theophylline on admission to intensive care units, in patients on long term treatment with oral Theophylline and/or had previously received an intravenous injection (IV) of Aminophylline. In 72 patients [52 chronic airflow obstruction (or BPCO), 21 asthmatics] admitted for acute respiratory insufficiency (IRA) the blood level of Theophylline on admission to intensive care (T0) was determined systematically, and was in the toxic range (greater than 29 mg/l) in 17% of cases (12/72). In patients with T0 greater than 20 mg/l, repeat measurements of Theophylline clearance were carried out: 12 hours after admission by studying the fall in plasma levels (Cl1), then after 8 days (+/- 5) while perfusing IV Aminophylline at a constant flow (Cl2) and finally after changing over to slow release oral Theophylline 18 days (+/- 10) after admission (Cl3). Cl1 was less than 35 ml/kg/h in 9 patients (group I) and greater than 55 ml/kg/h in 3 patients (group II). All the patients in group I were on oral Theophylline in a dose which was not excessive (mean 10.6 +/- 3.3 mg/kg/24 h) and only one patient received an injection of IV Aminophylline. In group I, the clearance of Theophylline was very low initially (Cl1 = 18.6 +/- 9.6 ml/kg/h) and finally rose (Cl2 = 34.7 +/- 14 ml/kg/h p less than 0.02; and Cl3 = 46.9 +/- 24 ml/kg/h p less than 0.02).(ABSTRACT TRUNCATED AT 250 WORDS)