Twenty patients receiving recombinant DNA gamma interferon were prospectively assessed for cardiac rhythm disturbances. All patients were evaluated with baseline electrocardiograms, pretreatment ambulatory monitoring and ejection fraction determination. Each patient was then monitored continuously during drug administration. Quantitative ventricular ectopy was not increased, nor were average heart rate, maximal heart rate, or quantitative supraventricular ectopy when comparing baseline to during therapy parameters. Complex cardiac ectopy and noteworthy cardiac events (NCE) were defined and identified in 2/20 (10%) patients pretreatment, and in 8/18 patients (44%) with normal baseline tracings during treatment (p = .02). This difference was not apparent when corrected for total days monitored pre- and post treatment (p greater than .2). These consisted predominantly of nonsustained short duration ventricular tachycardia (seven of eight patients). We conclude that life threatening cardiac events are uncommon with gamma interferon therapy.