The paper presents a literature review of research findings on acute treatment in psychiatric day hospitals, and outlines the design of an ongoing multi-site study on the effectiveness of acute day hospital treatment. The review is based on relevant older publications and recent randomised controlled trials comparing day hospital treatment with inpatient care. The findings suggest that (a) a wide range of patient groups are cared for in day hospitals, (b) both patients and their relatives assess day hospital treatment favourably, (c) treatment in day hospitals is at least as effective as inpatient care, and (d) day hospital treatment may lead to significant cost savings as compared to in-patient care. There are, however, a number of important questions that cannot be answered on the basis of the existing literature, and further research on the topic is warranted. The EDEN-study (European Day Hospital Evaluation) is a randomised controlled trial comparing day hospital treatment with conventional in-patient care and following the same protocol in five European countries (Czech Republic, Germany, Poland, Slovakia, and United Kingdom). Aims and methods of the study are presented.