Fifty consecutive patients were studied prospectively to assess the effects of a continuous intravenous infusion of midazolam hydrochloride for sedation in patients requiring intensive care. Patient comfort was acceptable in all patients. However, to maintain the same degree of sedation it was necessary to increase the daily dose of midazolam indicating that benzodiazepine tolerance may have been developing. The time taken to awaken following cessation of a midazolam infusion was prolonged in some patients. In those patients with renal failure the mean (+/- SD) value was 44.6 +/- 42.5 h compared to patients without renal failure in whom it was 13.6 +/- 16.4 h (P less than 0.01). Two patients with combined hepatic and renal failure took 124 and 140 h to awaken. Continuous intravenous infusion of midazolam offers good patient comfort but increasing dose requirements in critically ill patients may lead to drug accumulation and delayed awakening. The risks of cumulation may be increased if the drug is given by continuous infusion for prolonged periods without intermittent assessment of the patient's conscious state.