This article deals with the issue of diagnosis and qualification of patients for group psychotherapy. Meticulously conducted diagnosis and appropriate preparation for the beginning of treatment in a group reduces the risk of sudden resignation, increases the effectiveness of therapeutic interventions, gives the patients a greater sense of security, especially in the initial period of therapy. It also enables therapists to predict the patient's functioning in relations. In the literature we can find information about disqualifying factors as well as features favoring and facilitating the use of group therapy, but in reality qualification decisions are rarely clear and simple. The first part of the article reflects on the dilemmas related to the qualification of patients with various additional difficulties and dysfunctions, often described as excluding from group therapy. Next, reference is made to the importance of the diagnosis of mentalizing processes and personality assessment as significant predictors of functioning in a therapeutic group. Finally, the importance of diagnosis for the description of the subjects' problems and their preparation for psychotherapy, which is crucial for further treatment, was considered.