Tests for the detection of several types of cancer (multi-cancer screening) represent a new paradigm in cancer screening, where a single test is used for the simultaneous screening of many types of cancer. These tests raise important ethical questions regarding their development, evaluation and possible implementation. Ethical principles include beneficence and maleficence (proportionality), autonomy and justice. Proportionality requires that the benefits outweigh the harms, but current MCED tests do not meet these requirements due to limited validity and potential harmful consequences at the individual and societal levels. To respect autonomy, it is necessary to provide meaningful choices and understandable information. Distributive justice requires at least fairness in the implementation and costs of screening. The article was published in Nature Medicine online on January 5, 2026.[1]