INTRODUCTION: Critical intersectorial problems of co-operation are revealed through an organizational psychological analysis focusing on intergroup dynamics. The aim of the paper is to examine and improve the intersectorial co-operation in psychiatry and present a new trio-interview design that includes patients as well as staff. MATERIAL AND METHODS: Results from a follow-up study investigating the co-operation between Psychiatric Center Amager (PCA) and socialpsychiatric institutions (SPI) in Copenhagen is presented and analysed. The study was based on data gathered via structured interviews with patients and staff from both sectors (n = 60 in 2005, n = 87 in 2007), employing quantitative as well as qualitative methods. The newly developed trio-interview design with informant-triads for studies of the quality of intersectorial co-operation in psychiatry was applied to the study. RESULTS: Quantitative data from the patients showed that the co-operation was generally considered to be unsatisfactory in 2005 and 2007. SPI satisfaction estimates concerning the latest hospitalization period were higher in 2007 than in 2005. Staff from both sectors estimated that co-operation was significantly higher during 2007 than during 2005. Qualitative data showed that in 2007, staff was the decisive factor. CONCLUSION: The co-operation was estimated to be unsatisfactory in 2005 and 2007. Measures to strengthen intersectorial co-operation were implemented in 2006, but had limited effect.