The frequency and types of adverse reactions (ARs) occurring in hospitalized patients receiving furosemide were surveyed prospectively in a four-year study at the José Joaquin Aquirre Hospital in Santiago, Chile. A team of clinical pharmacists and clinical pharmacologists monitored 533 patients receiving furosemide and recorded patient characteristics, laboratory test results, drugs administered, and suspected ARs. Definite or probably furosemide-induced ARs were detected in 220 (39.8%) patients. No ARs were fatal but 7.6% of patients, all having cirrhosis of the liver, had severe ARs. Of the ARs recorded, 95.2% were dose-related. The most common ARs were electrolyte disturbances (23.5% of patients), extracellular volume depletion (9.0% of patients) and hepatic comma (3.6% of patients). Total and daily furosemide doses, lengthened hospitalization and hepatic disease were significantly associated with the frequency of ARs (p less than 0.001). The dosage of furosemide did not account for the increased frequency of ARs in patients with hepatic disease. The frequency of hypokalemia was not reduced significantly when furosemide was administered with potassium chloride or potassium-sparing diuretics (p less than 0.05). The frequency of severe furosemide-induced ARs is low, but ARs are more common in patients with cirrhosis of the liver. These patients should be closely monitored for furosemide-induced ARs.