Co–Cr– and Co–Cr–Mo-based alloys are commercially used in the industry especially for high wear resistance and superior chemical and corrosion performance in hostile environments. These alloys were widely recognized as the important metallic biomaterials. Here, the first development of Co–Cr–Mo–Fe–Mn–W and Co–Cr–Mo–Fe–Mn–W–Ag high-entropy alloys (HEAs) based on Co–Cr–Mo metallic biomaterials is reported. Ingots of six-component Co2.6Cr1.2Mo0.2FeMnW0.27 (Co41.5Cr19.1Mo3.2Fe16Mn16W4.3, at%) HEAs with a minor σ phase and of seven-component Co4.225Cr1.95Mo0.2FeMnW0.2Ag0.5 (Co46.6Cr21.5Mo2.2Fe11Mn11W2.2Ag5.5, at%) and Co2.6Cr1.2Mo0.1FeMnW0.1Ag0.18 (Co42.1Cr19.4Mo1.6Fe16.2Mn16.2W1.6Ag2.9, at%) HEAs without an σ phase were fabricated. The alloy was designed by a taxonomy of HEAs based on the periodic table, a treelike diagram, predicted phase diagrams constructed by Materials Project, and empirical alloy parameters for HEAs. The σ phase formation prevented the formation of solid solutions in Co–Cr–Mo-based HEAs without a Ni element. The σ phase formation in as-cast ingots was discussed based on the composition dependence and valence electron concentration theory.