The interrogative warum connected with a bare infinitival VP forms a specific kind of root structures in German. wh-root-infinitives in general are subject to certain syntactic and interpretative restrictions. The subject argument of the infinitival verb cannot be expressed by a lexical NP; it is identified with the speaker, instead. Due to the lack of finiteness these infinitives are restricted to a non-past, prospective, modal interpretation. warum-infinitives share these properties. But differing from other instances of wh-infinitives they are systematically ambiguous, as they provide a second reading with a non-prospective interpretation referring to a present or past event in which the subject argument is identified with the addressee. It will be argued that the availability of this second reading has to be ascribed to a pragmatic inference which refers to presuppositions of questions in general and of causal questions in particular.