Despite evidence for its efficacy, diagnosis-specific psychoeducation is not routinely applied. This exploratory randomized controlled trial analyses the efficacy of an easily implementable bifocal diagnosis-mixed group psychoeducation in the treatment of severe psychiatric disorders regarding readmission, compliance and clinical variables, for example global functioning. Inpatients of the Psychiatric Hospital of the University of Basel (N = 82) were randomly assigned to a diagnosis-mixed psychoeducational (PE) or a non-specific intervention control group. Relatives were invited to join corresponding family groups. Results at baseline, 3- and 12-month follow-ups are presented. Better compliance after 3 months and a lower suicide rate were significant in favour of PE. For most other outcome variables, no significant differences, however advantages, in PE were found. In summary, it can be concluded that diagnosis-mixed group psychoeducation is effective in the treatment of severe psychiatric disorders. The effects can be classified as induced by distinctive psychoeducational elements. Findings similar to those on psychosis-specific programmes justify clinical application and further investigation.