A correction of background activity that considers the heart volume is introduced in radionuclide equilibrium cardiography. Left ventricular ejection fraction (LVEF) was determined in 19 patients with ischaemic heart disease by multigated equilibrium scan with 99Tcm-labelled red blood cells (99Tcm-RBC). Assuming that the majority of background activity in the ventricular and periventricular regions of interest arises from 99Tcm-RBC in lung tissue, a ventricular/periventricular background ratio was calculated from the distribution of 99Tcm-labelled macroaggregated albumin. The mean relative distribution of pulmonary 99Tcm-RBC activity in the ventricular region of interest was 0.590 (range 0.279-1.061) compared with the periventricular region. The modified background correction resulted in a mean LVEF of 43.0% (range 12.9-67.6%) compared with 46.8% (range 10.9-67.8%) with the standard correction, p less than 0.005. Since no attempt was made to calculate the proportion of 99Tcm-RBC activity arising from the myocardium and thoracic wall, this difference represents an upper limit of the general overestimation of LVEF obtained by the standard calculation. A further decrease in LVEF was seen when no background correction was applied. The present results suggest that standard LVEF calculation, by neglecting the background volume displaced by the cardiac blood pool, results in a slight overestimation of LVEF.