Rare disease organizations are issue-based (people gather to struggle with common problems that they want to solve together) while coalitions of such organisations tend to be rarity-based (organisations come together because their members address rare disorders). A series of 61 interviews with patients, relatives, volunteers and employees in 8 French associations identified three solidarity principles for these coalitions: (1) the "one for all" principle, where any advancement for one disorder will benefit many others; (2) the "every man for himself" principle, where one should support their own rare disorder organisation; (3) the "all for one" principle, where solidarity is moral. None of these principles is satisfying. Gathering people with/or related to rare disorders with others addressing the same issues, i.e. an issue-based approach in ad-hoc coalitions, has the potential to be strong and cohesive. E.g.: coalitions of rare disease associations with Aids or cancer associations for access to insurance or credit.