In four patients (a 54-year-old man and three women aged 57, 60 and 65 years, respectively) with colorectal carcinoma and no obvious cardiac abnormality anginal symptoms and ECG changes occurred during chemotherapy with 5-fluorouracil at a dose of 400 and 600 mg/m2. The ECG changes consisted of descending ST depressions with preterminally negative T waves, terminally negative T waves, ventricular extrasystoles and sinus tachycardia with intermittent atrial fibrillation. The signs first appeared between the second and fourth day of treatment; in two patients they improved with glyceryl trinitrate. A few days after 5-fluorouracil had last been administered all ECG changes had disappeared. The causes of cardiotoxicity of the drug remain unknown.