Objective: To assess the burden on relatives as well as their mental well-being within the context of a randomized controlled trial on the effectiveness of acute psychiatric day hospital treatment as compared to inpatient treatment. Method: The study was conducted at the Department of Psychiatry and Psychotherapy, Dresden University of Technology. A sample of 95 relatives was assessed at admission and after four weeks of treatment using the Involvement Evaluation Questionnaire (IEQ.) and the General Health Questionnaire (GHQ-28), the period to be rated being the last four weeks prior to assessment. Results: Relatives reported a mean level of burden, which for both settings decreased during the first four weeks of treatment. With respect to this period, burden on relatives of day hospital patients did not differ from that on relatives of inpatients. The relatives' mental well-being was markedly impaired, and only that of the inpatients' relatives did slightly improve during treatment. Conclusions: Treating acute mentally ill as day hospital patients does not result in greater burden on relatives compared to treating them as inpatients. Independently from treatment setting, relatives of psychiatric patients should be actively approached and offered information, coping strategies, and help. Further research should include qualitative methods.