BACKGROUND: We set out to investigate the reliability and validity of the Japanese version of the CDC core healthy day measures assessing health-related quality of life (CDC HRQOL-4) in Japanese workers. METHODS: This cross-sectional study was conducted among 1360 Japanese workers of one Japanese company located in Kyushu. Cronbach's alpha was calculated to evaluate the internal consistency of CDC HRQOL-4 items. The concurrent validity was tested by assessing whether the CDC HRQOL-4 items correlated well with the corresponding domains of the SF-8 and the overall WFun score. The construct validity was tested by assessing the ability of the CDC HRQOL-4 to discriminate groups with and without any current disease, pain, mental problem, and sleeping disorder. RESULTS: Cronbach's alpha for three of the four CDC HRQOL-4 items was 0.80, which is greater than the minimal standard (0.70) recommended for internal consistency reliability. Correlation coefficients ranging from 0.25 to 0.67 were obtained between CDC HRQOL-4 items and the SF-8 domains and WFun score. Workers with any current disease, mental problem, pain or sleeping disorder reported higher numbers of unhealthy days, and a higher odd of poor health than those without such problems. Japanese version of the CDC HRQOL-4 shows a good concurrent validity with the SF-8 and the WFun tool and good construct validity among Japanese workers in the current study. CONCLUSIONS: Japanese version of the CDC HRQOL-4 is a reliable and valid instrument that may be used to assess overall health and physically and mentally unhealthy days in Japanese workers.