The magnitude of acute arrhythmogenicity of doxorubicin administration has not been characterized. In this study the type and frequency of cardiac arrhythmia is determined in the first hour and first 24 hours following doxorubicin given as a 10-minute infusion. Twenty-nine patients with diverse malignancies were studied with Holter monitors on 33 postdoxorubicin days. Control recordings were made on days remote from doxorubicin treatment. The frequency of arrhythmia that could be attributed to doxorubicin was low: 3% of studies in the first hour postinfusion and 24% of studies in 1 to 24 hours postinfusion. The most commonly seen arrhythmia was an increased frequency of ventricular premature beats. Arrhythmia following doxorubicin infusion is rare in the first hour and more common in the remainder of the first 24 hours postinfusion. There are no acute or long-term adverse consequences related to the appearance of arrhythmia due to doxorubicin.