Clinical Dementia Rating (CDR) scale and Global Deterioration Scale (GDS) are commonly used to measure the severity of dementia. However, no specific rules are available to convert the scores of CDR into those of GDS and vice versa. Using a semi-structured interview, two examiners independently rated CDR and GDS in 78 patients with dementia and 34 controls. Regression analysis showed a curvilinear relationship between CDR and GDS. This curve may provide a rule to interchange the scores of GDS and CDR (or Sum of Boxes of CDR).