Partial volume effect (PVE) and tagging inhomogeneity are two major causes of artifacts in electronic cleansing (EC) for fecal-tagging CT colonography (CTC). Our purpose was to develop a novel method called "virtual tagging" for electronic cleansing in dual-energy fecal-tagging CTC. A three-material decomposition scheme was first applied in dual-energy CTC to decompose each voxel into a mixture of air, soft tissue, and iodine-tagged fecal material. The entire colonic lumen was then marked by virtually tagging the mixture portion of luminal air at each voxel. As a result, colon lumen including air and tagged materials was segmented and subtracted by their high values in virtually tagged images. Our virtual tagging scheme provides a cleansed colon that is free from artifacts caused by the PVE at air-tagging mixture and inhomogeneous tagging.