The hemodynamic background associated with the occurrence of paroxysmal atrial fibrillation and flutter (PAF), in patient with acute myocardial infarction (AMI) were evaluated. Sixty-seven of 381 consecutive AMI patients (17.6%) were noted to have PAF in the acute phase of infarction. These 67 patients with PAF (group 1) were compared with 60 randomly selected patients without PAF (group 2). The hospital mortality rate was 25.4% in group 1, and 11.7% in group 2 (p less than 0.01). The hemodynamic variables measured before the onset of PAF in group 1, showed significantly more unfavorable values than those in group 2, which were measured at the time of admission. The 67 patients in group 1 were divided into 50 patients who survived (group S) and 17 patients who died in the hospital (group D). The hemodynamic status in group D demonstrated significantly larger deterioration before the onset of PAF than in group S. Hemodynamic variables were compared before and during PAF in groups D and S, cardiac index (CI) decreased significantly, and stroke index (SI) decreased by 46% in group D, with no decrease in CI and less decrease in SI (28. p less than 0.05) in group S. In conclusion, not only the occurrence of PAF, but the prognosis of patients with PAF is dependent on the severity of hemodynamic disturbance imposed by AMI. Atrial contribution to ventricular filling has great importance in the maintenance of the cardiac output in this patient population.