To evaluate the prognostic value of the left ventricular response to isoproterenol infusion in patients with dilated cardiomyopathy (DCM), 25 patients, 17 men and eight women, were studied. According to responses of left ventricular function to isoproterenol (0.02 microgram/kg/min), the patients were classified in two groups: the normal response group, in which fractional shortening increased by more than 10% (n = 10); and the low response group, in which fractional shortening increased by 10% or less (n = 15). A follow-up spanning four to 40 months with an average of 21 months disclosed that six patients died, two deteriorated, and six had no change in the low response group, while seven patients were improved, three stabilized, and no one deteriorated or died in the normal response group. There was a difference in the clinical courses of the two groups. Thus, the left ventricular response to isoproterenol proved useful in predicting the course of DCM.