Policy choices are often regarded as compromises between public authorities and interest groups, even when the use of experts is required because of their technical nature. In this article, it is shown that expertise cannot be considered as a ‘passive’ resource in the hands of interest groups, but as a relatively autonomous field with specific rules that impacts policy outcomes. Thus, the convergence of positions between some experts and some interest groups needs investigation and explanation. This contention is illustrated through the analysis of the codification of occupational diseases in France. This process involves a deep medical expertise and has sparked a very intense controversy between labour unions and employers’ organizations.