The determinants of change in left ventricular (LV) ejection fraction (EF) over time in patients with impaired LV function at baseline have not been clearly established. Using a nuclear database to assess changes in LV function over time, we included patients with a baseline LVEF of 25% to 40% on a gated single-photon emission computed tomographic study at rest and only if second-gated photon emission computed tomography performed approximately 18 months after the initial study showed an improvement in LVEF at rest of ≥10 points or a decrease in LVEF at rest of ≥7 points. In all, 148 patients qualified for the EF increase group and 59 patients for the EF decrease group. LVEF on average increased from 33 ± 4% to 51 ± 8% in the EF increase group and decreased from 35 ± 4% to 25 ± 5% in the EF decrease group. The strongest multivariable predictor of improvement of LVEF was β-blocker therapy (odds ratio 3.9, p = 0.002). The strongest independent predictor of LVEF decrease was the presence of a permanent right ventricular apical pacemaker (odds ratio 6.6, p = 0.002). Thus, this study identified β-blocker therapy as the major independent predictor for improvement in LVEF of ≥10 points, whereas a permanent pacemaker (right ventricular apical pacing) was the strongest predictor of a LVEF decrease of ≥7 points.