Introduction Over the past decade computed tomography of the breast has been developed by several research groups in the USA and in the European Union to overcome the limitations of overlapping tissue in mammography. Fibroglandular tissue has a much higher attenuation coefficient than fatty tissue at all energies but carcinomas have a slightly higher attenuation coefficient below 31 keV. In this study a virtual multi-energy CT unit is used to explore the differentiation of breast tissues at different energies by means of Monte Carlo simulations. Materials and methods The egs_cbct Monte Carlo code was used to model a virtual CT which was benchmarked against the Aquillion 16 LB CT unit by means of a RMI phantom. Breast simulator software was used to create phantoms of different breast tissue. Different energy spectrums were used to investigate the differentiation between the different tissues. Results The Hounsfield units (HUs) of the simulated RMI phantom corresponded well with that of the Aquillion 16 LB CT scans. The differentiation between breast tissues could be seen at all energies. Lower energies resulted in a better distinction between cancerous tissue and normal tissue due to the attenuation coefficient dependency on the energy. A suggestion of energies was made that could be used for photon-counting breast CT. Conclusion It is feasible to use multi-energy CT for the differentiation of breast tissue. This can aid in the detection of cancerous tissues especially in dense breast which is difficult with two-dimensional mammography.