PURPOSE: To determine the optimal window settings for monochromatic images with various energy levels in single-source dual-energy computed tomography (DECT) of the abdomen. MATERIALS AND METHODS: Two hundred consecutive patients underwent contrast-enhanced DECT to screen tumor metastases and/or recurrences after surgery for malignant tumors. Two independent radiologists reviewed eight energy levels (40, 45, 50, 55, 60, 65, 70, and 75 kilo-electron volts [keV]) of the portal venous phase monochromatic images. For each keV image, the radiologists adjusted the window level (WL) and width (WW) using settings of 40 HU of WL and 350 HU of WW on 65 keV images as a reference and recorded these values. After removing the top and bottom 5% of the data, the optimal WL and WW in each energy levels were obtained by rounding the median values. In 7 of 200 patients with a total of 23 liver metastases, the tumor-to-liver contrast-to-noise ratio (CNR) was calculated and compared among the energy levels. RESULTS: The optimal WLs and WWs at each energy level were as follows: 40 keV, 140 and 680 HU; 45 keV, 110 and 570 HU; 50 keV, 90 and 490 HU; 55 keV, 70 and 440 HU; 60 keV, 60 and 390 HU; 65 keV, 40 and 350 HU; 70 keV, 30 and 320 HU; and 75 keV, and 20 and 300 HU. The best CNR was obtained using 65 keV images (P < 0.001). CONCLUSION: We clarified the optimal WL and WW available for the preset window settings in abdominal DECT.