The evaluation of outcomes in mental health care embraces evaluation, quality assurance, and progress measurement of treatments. The Clinical Outcome in Routine Evaluation - Outcome Measure (CORE-OM) is an outcome focused self-assessment instrument, comprising 34 items covering four scales well-being, problems, functioning, and risk. The questionnaire has been translated into 52 languages, including Russian. Despite its broad application, the dimensionality of the CORE-OM deserves some further research. Thus, the present study examines the dimensionality of the Russian CORE-OM using the multidimensional random coefficients multinomial logit model (MRCMLM) based on data of N = 240 patients. The results indicate the need for further research on factorial structure and response formats of the CORE-OM. In addition, differential item functioning was found for gender and diagnostic groups, suggesting separate test norms. Again, the MRCMLM and the Test Analysis Modules (TAM) package have proven valuable tools for investigating a questionnaire's psychometric properties.