 The patient-rated Core Outcome Measures Index (COMI) assesses the multidimensional impact of back problems on the sufferer. The brevity and comprehensibility of the tool make it practical for use in clinical and research settings. Although the COMI has been cross-culturally adapted in various languages worldwide, there is currently no Japanese version. The aim of this study was to develop a Japanese version of the COMI by: (1) performing a cross-cultural adaptation of the English version and (2) evaluating the psychometric properties of the Japanese version of the COMI in Japanese volunteers with chronic back problems.