Left ventricular ejection fraction was measured by radionuclide left ventriculography before and immediately after hemodialysis in 20 uremic patients, 11 of whom presented with congestive heart failure. Ejection fraction and contraction were normal in 15 patients (Group A), six of whom had signs of congestive failure; they were abnormal in five patients (Group B), all of whom were in clinical heart failure. Mean arterial pressure and body weight decreased by a similar amount after dialysis in both groups, and heart rate did not change. In Group A ejection fraction was unchanged by dialysis (0.63 +/- before vs. 0.62 +/- 0.09 after) (mean +/- S.D.), but in Group B it was improved significantly (0.32 +/- 0.04 before vs. 0.44 0.08 after) (P less than 0.01). In three patients in Group B cardiomegaly and ejection fraction returned to normal with long-term hemodialysis. In end-state renal failure, radionuclide left ventriculography can separate patients with circulatory congestion due to fluid overload from patients with left ventricular dysfunction in whom hemodialysis can provide immediate and long-term improvement.