Purpose <p>To clarify the usefulness of electron density (ED) using dual-energy CT (DECT) parameters for predicting treatment response in patients with pulmonary embolism (PE).</p> Materials and methods <p>The study population comprised 30 patients with PE (49 thrombi) who underwent pretreatment DECT. The study coordinator diagnosed PE using contrast-enhanced CT (CECT) as the gold standard and annotated the location of thrombi on CECT prior to the DECT image analyses. CT attenuation values on conventional 120 kVp, 40&#xa0;keV, and 70&#xa0;keV virtual monochromatic (VM) images; effective atomic number; and ED of pretreatment pulmonary thrombi were measured on unenhanced CT. Thrombi were classified into dissolved and residual groups according to the findings of posttreatment follow-up CT. DECT parameters were compared between the two groups using the Mann–Whitney <i>U</i> test. For statistically significant parameters, receiver-operating characteristic (ROC) analysis was used to evaluate their performance for differentiating two groups. Diagnostic accuracy for predicting treatment response in patients with PE was determined by calculating the area under the ROC curve (AUC).</p> Results <p>ED values, CT values on conventional 120 kVp imaging, and those on 70&#xa0;keV VM imaging were significantly higher in thrombi in the dissolved group than the residual group (<i>p</i> &lt; 0.001, <i>p</i> = 0.012, <i>p</i> = 0.009, respectively). AUC values for predicting dissolution response by ED, conventional 120 kVp imaging, and 70&#xa0;keV VM imaging (cut-off value, 3.49 × 10²³/cm³, 53.4 HU, and 50.7 HU, respectively) were 0.856, 0.744, and 0.755, respectively. AUC was significantly higher for ED than for conventional 120 kVp imaging and 70&#xa0;keV VM imaging (<i>p</i> = 0.032, <i>p</i> = 0.016).</p> Conclusions <p>ED derived from unenhanced DECT may help predict therapeutic efficacy in patients with PE.</p>