JCOG (Japan Clinical Oncology Group) was started in 1990 and is the only governmental clinical trial group in Japan. The ultimate purpose of JCOG is to establish gold standard therapies for each tumor type based on scientifically and ethically scheduled investigator-initiated clinical trials. JCOG is composed of various committees for auditing and management, a data center and clinical study groups. The steering committee meeting is held four times a year and decides the missions, principles and policies of JCOG, the exchange of JCOG members and the approval of practical protocols for clinical trials. JCOG has 15 study groups and about 170 member institutions and hospitals. The number of nominated institutions is limited to keep high quality of JCOG. Inactive institutions have been replaced with other institutions by the steering committee. The number of patients accrued for each clinical trial is influenced by the number of active protocols in each trial group. Some groups are very active, while others have not yet completed any trials. JCOG conducts only investigator-initiated trials and so far has not been involved in any IND (Investigational New Drug) trials. Because JCOG does not receive any funding from pharmaceutical companies, the data are not biased by conflicts of interest. Since 1978, JCOG has conducted 216 clinical trials, the majority of which were Phase III trials. The results have been presented at regional and international scientific meetings and have been reported in mainly English language journals as original articles. Some have been published in journals with high impact factors, such as the New England Journal of Medicine, the Journal of the National Cancer Institute, and the Journal of Clinical Oncology. In addition to the evaluation of new anticancer drugs, radiation therapy and new surgical procedures have also been tested in randomized controlled trials. Extremely important data have been published by surgical groups. Some clinical trials have produced positive results, but others did not achieve their primary endpoints. In addition, the accrual of patients was sometimes so poor that the clinical trial had to be interrupted. In a few studies, an independent data monitoring committee suggested that patient accrual be stopped because of the high incidence of severe toxicity arising from treatment and unexpected negative or inverse results. Recently, the publication of such data has become extremely difficult because of the rapid increase in articles that are being submitted to popular journals. The editors of such journals have suggested that the rejection rate should be as high as 80–90% and that ‘Me too-type’ articles with no significant data and single-arm Phase II studies of standard therapy should be rejected because the cost of publication is becoming too high and too many articles are waiting for publication. As a result of this situation, data concerning negative results or interrupted clinical trials are often difficult to publish. Nevertheless, such information is very important to young active investigators who are developing new protocols for clinical trials. JCOG has a policy that the outcome of a clinical trial should be published in English once it has been approved by the protocol review committee. The Japanese Journal of Clinical Oncology has agreed to publish review articles for each clinical trial group, enabling ‘hidden data’ to become available. Consequently, the chairman of each group in JCOG has been asked to write a review article on their study group. In each review, the author has included all the clinical trials within their group that have been approved by the protocol review committee of JCOG. Readers should be able to recognize the development/refinement of each study group and understand the reasons for negative results and low patient accruals as well as the unexpected early termination of studies. Readers will also be able to understand the success rate of clinical trials. To complete each clinical trial, numerous specialists must join and collaborate with one other. Therefore, clinical trials with negative results and with low accruals should be avoided as much as possible. This series will provide important information regarding the writing of proper clinical protocols for clinical trials. The first review is written by Dr Takashi Onda, Gynecologic Oncology Division of National Cancer Center, on behalf of the chairman of the gynecological group. The group has published four articles reporting the results of the JCOG9412, 0206, 0602 and 0505 trials in the Int J Gynecol Cancer, Gynecol Oncol, Jpn J Clin Oncol and Jpn J Clin Oncol, respectively. Additionally, JCOG0102 and 0503 are introduced in the review article.