We describe four patients who suffered asystole, myocardial infarction (in two cases) and supraventricular tachycardia at the time of high-dose methylprednisolone infusion. One patient died during infusion. We measured the effects of methylprednisolone and placebo infusion on heart rate, blood pressure, and cardiac output in six stable renal allograft recipients. A significant rise in arterial pressure, caused by an increase in total peripheral resistance, occurred during methylprednisolone infusion. Possible mechanisms include potentiation by methylprednisolone of the vascular effects of the high plasma noradrenaline levels that are found in renal allograft recipients. We suggest that electrocardiograms and blood pressure measurements should be monitored during methylprednisolone infusion, especially in patients with preexisting cardiovascular disease.