Heparin is frequently used for the prophylaxis and treatment of deep venous thromboembolism and it induces hypoaldosteronism leading to hyperkalemia, an uncommon adverse effect. In an intensive prospective drug monitoring study, 154 inpatients at the Internal Medicine Unit of Hospital Sotero del Río, Santiago, Chile, received heparin in the period between March and November 1990. Mean age of the patients was 65.8 +/- 12.9 years and 56.5% were female. Twenty-one (13.6%) patients developed heparin-induced adverse reactions. Thirteen events were hyperkalemia, 7 ecchymoses and 1 hematuria. The monitoring team and attending physicians have agreed to classify 9 heparin-induced hyperkalemia cases as probable and the other 4 as possible. No adverse reaction was fatal but 8 of the patients had severe hyperkalemia. Almost all reactions were dose-related. Hyperkalemia was more frequent in patients with diabetes mellitus, metabolic acidosis and long-term heparin therapy. The frequency of hyperkalemia did not correlate with age, sex, renal impairment or with previous use of anti-inflammatory drugs, heparin or aspirin.