Operational diagnostic systems like ICD-10, Chapter V (F) are introducing the concept of comorbidity covering the symptomatology of patients with multiple descriptive diagnoses. Focusing on this concept, the relevance of rater variables in the diagnostic assessment with ICD-10 is studied. Based on data from the German ICD-10 Research Criteria Study in 11 centres of psychosomatic medicine, diagnostic ratings of 129 clinicians for 20 patients are discussed. Significant differences concerning the agreement of the clinicians with expert main diagnoses are correlated with their familiarity with DSM-III-R, the age of the clinicians and subjective ratings of the diagnostic process. Clinicians with a greater familiarity with DSM-III-R and ICD-10 and older diagnosticians with more professional experience in psychosomatic medicine show a better ability to cover comorbidity in their diagnostic assessments. Furthermore, behavioural therapists are assessing comorbidity more adequately than psychoanalytic therapists.