Arterial and pulmonary artery catheters were used to monitor the cardiopulmonary effects of recombinant interleukin-2 (rIL-2) given iv at a dose of 100,000 U/kg every 8 hours on days 1-5 to 10 patients with metastatic solid tumors. As anticipated, a severe capillary leak syndrome developed in all patients. Myocardial infarction (MI) occurred unexpectedly in three patients, as evidenced by a focal injury pattern on ECG and elevations of creatinine phosphokinase myocardial band fractions. All patients receiving rIL-2 exhibited major reductions in their left ventricular stroke work index (47 +/- 11 g.m/m2 to 29 +/- g.m/m2), an index of cardiac contractility. It remains uncertain whether the MIs were a byproduct of the capillary leak syndrome in patients with underlying coronary artery disease or whether rIL-2 directly or indirectly damages cardiac muscle.