Objectives. The efficacy of high dose furosemide as a continuous infusion was compared with a bolus injection of equal dose in patients with severe heart failure. Background. The delivery rate of furosemide into the nephron has been proved to be a determinant of diuretic efficacy in healthy volunteers. Methods. In a randomized crossover study we compared the efficacy of a continuous infusion of high dose furosemide (mean daily dosage 690 mg, range 250 to 2,000) versus a single bolus injection of an equal dose in 20 patients with severe heart failure. The patients received an equal dosage, either as a single intravenous bolus injection or as an 8-h continuous infusion preceded by a loading dose (20% of total dosage). Results. Mean (±SEM) daily urinary volume (infusion 2,860 ± 240 ml, bolus 2,260 ± 150 ml, p = 0.0005) and sodium excretion (infusion 210 ± 40 mmol, bolus 150 ± 20 mmol, p = 0.0045) were significantly higher after treatment with continuous infusion than with bolus injection, despite significantly lower urinary furosemide excretion (infusion 310 ± 60 mg every 24 h, bolus 330 ± 60 mg every 24 h, p = 0.0195). The maximal plasma furosemide concentration was significantly higher after bolus injection than during continuous infusion (infusion 24 ± 5 μg/ml, bolus 95 ± 20 μg/ml, p < 0.0001). Short-term, completely reversible hearing loss was reported only after bolus injection in 5 patients. Conclusions. We conclude that in patients with severe heart failure, high dose furosemide administered as a continuous infusion is more efficacious than bolus injection and causes less ototoxic side effects.