This paper presents the work that we have carried out in inves tigating the purpose of discourse structure forwhy-question answering (why-QA). We developed a system for answer- ing why-questions that employs the discourse relations in a pre-an notated document collection (the RST Treebank). With this method, we obtain a recall of 53.3% with a mean reciprocal rank (MRR) of 0.662. We argue that the maximum recall that can be ob tained from the use of RST relations as proposed in the present paper is 58.0%. If we dis card the questions that require world knowledge, maximum recall is 73.9%. We conclude that d iscourse structure can play an important role in complex question answering, but that more forms of linguistic processing are needed for increasing recall.