The assessment of diagnoses is a central issue in the treatment of patients with mental disorders. The aim of this study was to validate routine clinical diagnoses by means of a semi-standardized and structured interview. Semi-standardized and structured interview diagnoses were based on videos of 55 inpatients. The routine clinical diagnoses were given by therapists during the course of psychotherapy. Validation was carried out through proportional agreement, Cohen's kappa, Yule's Y, as well as the sensitivity and specificity of the diagnosis. Agreement rates between diagnoses given in semi-standardized structured interviews and those given in routine clinical assessment were low for the majority of specific disorders (e.g., major depressive disorder). Higher agreement rates were found for major diagnostic categories (e.g., eating disorders or adjustment disorders). In comparison to diagnoses according to structured interviews for inpatients with mental disorders, the validity of diagnoses given in routine clinical interviews is limited and should be improved.