There are differing opinions on how regulations of the World Health Organization should be classified under traditional theories of the sources of international law. Are they treaties, fully subject to the law of treaties contained in the Vienna Convention and its counterpart in customary international law? Or should they be considered part of the secondary law of an international organization as a manifestation of the law-making authority of WHO, akin to something like a binding Security Council resolution or a European Union regulation, but sui generis to WHO’s legal structure? This article aims to unpack the question of the legal nature of WHO regulations and its consequences. It describes the concept of regulations in the historical and constitutional context of WHO, discusses the practical significance of the legal nature of regulations and examines substantive and formal criteria to explore whether we should treat regulations as treaties or secondary instruments within the legal order of WHO.