This paper investigates the emergence and evolution of Portuguese caso (< Lat. CASUS) in conditional contexts. As in similar cases in other European languages, conditional constructions involving caso emerged in medieval language in juridical texts. The paper examines the first attestations of such constructions and shows how they allow the further evolution of caso in Portuguese into a conditional conjunction. This evolution can be initially identified in Brazilian Portuguese from where it possibly spread to the European variety. Other evolutions such as the emergence of a noun acaso ‘coincidence’ or ‘fate’ and a modal particle acaso (in both Portuguese and Spanish) are also considered. The theoretical aim of the paper is to show the interrelation between grammaticalization and discourse traditions in the sense of Koch (1987, 1997): innovations emerge in particular textual environments, not only in the language as an abstract entity, and they may spread from their original textual tradition to others. The main claim of the paper is thus that the widening of the scope of discourse traditions should be considered as a general parameter of grammaticalization processes.