Computer-based clinical decision support systems (CDSSs) are still today not in widespread use, although there has been extensive research and development for several decades. We have, based on the literature of the last years, summarised some of the problems that may lead to low user acceptance and that should be addressed more extensive in future developments of CDSSs. We describe different aspects of the usefulness of CDSSs according to the elements (relevance, validity, and work) of a formula of usefulness of information, and, in a short section, refer to the discussion about clinical guidelines.