Looking at the sometimes disappointing under-use of the psychiatric case register in Dutch mental health care, this article analyses the difficult relation between policy- making and research in the field of mental health. Three factors which influence the relevance of research for policy are discussed: (1) characteristics of the policy process, (2) types of research, (3) communication between research and policy. It seems that mental health policy seldom reflects the assumptions of the so-called comprehensive or synoptic policy process. Therefore, research that is oriented only on this policy model will be underused if policy-making includes more elements of the so-called incremental policy process. It is argued that the chances of use of research in policy wil be higher according if the characteristics of the specific policy process, wherein research has to play a role are taken into account. The implications of these conclusions for the psychiatric case register are discussed.