We thank Dr Thomas Erren and his colleagues 1 for their comments on our recent article on prostate cancer risk among shift workers. 2 In October 2007, an International Agency for Research on Cancer panel concluded that shift work involving circadian disruption is probably carcinogenic to humans.3 In modern society, shift work is inevitable and essential; the prevalence of night shift work among workers is reported to be 17.9% in Japan and 17.3% in European Union member states.3,4 Given the significant impact of cancer on health and the widespread prevalence of shift working, an increased cancer risk among shift workers is a public health concern, although only limited supporting epidemiological evidence exists.As Erren et al. described , the first advantage of our study was the high-quality exposure information. Detailed information of shift systems was not reported in prior related studies. Different shift schedules might differentially impact health, thus, the search for a favorable shift system is of high importance. Subjects of our study had engaged in a counterclockwise three-shift work schedule, which is suggested to have a more negative impact on health compared with clockwise shift rotation. However, as suggested by Erren et al., we did not emphasize this point in our paper, because other types of shift systems were not evaluated; thus, the study could not independently contribute to resolving the issue. We do, though, agree that future work should provide detailed information regarding the type of shift schedule, as far as possible, to help address the impact of different shift systems on health.Our study provided additional evidence regarding prostate cancer risk among shift workers, although the result was not statistically significant. The mutual background of the authors is occupational medicine and we regard that unnecessary disturbance of the workplace should be strictly avoided; thus, we had an earnest discussion as to whether to publish this "statistically insignificant" (i.e. inconclusive) result. Despite our reservations, we decided to publish our study for the following reasons. First, the authors regarded that this study included the best available evidence to date in terms of exposure assessment, which is the first limitation of the previous related studies. Second , this study could be a good candidate for future meta-analysis. Finally, there are numerous workers worldwide who engage in shift work, and evidence should be made available for occupational health professionals to support their interpretation and decisions regarding cancer risk among shift workers. We hope that our publication will serve as a primer for future related studies, particularly in industrial settings.It warrants mention that the incidence rate of prostate cancer in Japan is increasing, although it remains low compared with the USA and European Union member states. This could be one of the reasons for the limited numbers of cancer cases among our study population; however, the relatively young age of subjects is like...