["Bioethics, EarlyView. ", "\nABSTRACT\nRarity provides a challenging case for contemporary priority setting. On the one hand, many philosophers and economists argue that rarity has no inherent moral value, and thus that rare diseases merit no special treatment in priority setting decisions simply because they are rare. On the other hand, existing priority‐setting practices demonstrate a higher willingness to pay for rare disease treatments. We argue that special priority for rare diseases might be justified on egalitarian grounds. Specifically, we develop and defend what we call the “bad numbers luck” argument for prioritizing rare diseases. This is a variant of the luck egalitarian idea of bad price luck. We conclude by discussing how higher willingness to pay, adjusted priority‐setting processes, and a broader societal focus on rare diseases could address such injustices. Recognising bad numbers luck clarifies the relationship between fairness and efficiency in health care priority setting.\n"]