AIMS: To explore whether the diagnostic homogeneity in a daily, routine clinical activity changed visibly over two historical periods (the ICD-8 and the ICD-10 era) across and within five psychiatric in-patient clinics. METHODS: In this register study, we analyzed the discharge diagnoses from five university-affiliated departments of psychiatry in Denmark in two time periods: 1980-1985 (ICD-8) and 2001-2010 (ICD-10). RESULTS: The synchronic inter-departmental diagnostic differences did not decrease in the ICD-10 era compared with ICD-8 era. Nor did the diachronic stability within each department become more homogeneous. CONCLUSION: The diagnostic variability reflected by the diagnostic differences between the departments and by the diagnostic homogeneity within each department remained similar in the two historical periods with no evidence of an increased homogeneity of diagnostic habits after the introduction of the ICD-10. LIMITATIONS: There is a myriad of variables that affects the diagnostic variability over time that we were not able to control.