Launched in 2000 and maintained by the NLM, ClinicalTrials.gov is an online database of clinical research studies, key characteristics of their design, and information about their results. The purpose of the database is to improve transparency in clinical research by providing information about clinical studies to research participants, the public, researchers, and healthcare professionals.

The registry was developed to help ensure that information about clinical research is shared with the public in a timely manner and to enable a full understanding of the effectiveness of clinical interventions. Public registration of clinical trials also helps researchers identify knowledge gaps, avoid duplicating research efforts, and plan and design future studies (Califf et al 2025). Moreover, because peer-reviewed journals often reject reports of studies with negative results, small studies, and trials stopped early, ClinicalTrials.gov is a critical dissemination strategy for filling these gaps (Piller 2015).

ClinicalTrials.gov now provides information about more than 500,000 clinical studies, from observational studies and small feasibility studies to large randomized clinical trials (Califf et al 2025).

Numerous federal regulations, agency policies, and professional guidelines require the submission of key information about clinical studies to ClinicalTrials.gov (Zarin et al 2020):

• The Food and Drug Administration (FDA) Amendments Act and associated regulations require the registration of every clinical trial of an FDA-regulated drug, biologic, or device product, not including phase 1 trials and small feasibility studies, within 21 days after the enrollment of the first trial participant.
• NIH-funded clinical trials of any type, including phase 1 trials and feasibility studies, must be registered within the same time frame.
• The ICMJE calls for registration of all clinical trials on or before the enrollment of the first participant.
• Clinical trials funded by the Department of Veterans Affairs Office of Research and Development and studies funded by the Patient-Centered Outcomes Research Institute (PCORI) must be registered before the enrollment of the first study participant.

In 2008, ClinicalTrials.gov was expanded to enable the reporting of results of registered clinical trials. This change, made in response to the FDA Amendments Act, represented an important step in fulfilling the ethical obligation of transparency to research participants about the results of their participation. Federal regulations require the submission of summary results for registered studies within 1 year after the study is completed (with some delay permitted under certain conditions).

Results reporting in ClinicalTrials.gov consists of the following elements:

• Summary of trial participants’ flow through the trial by study group
• Summary of all baseline measures, including participant demographic characteristics, used in the primary outcome analysis
• Summary of aggregate results for all prespecified primary and secondary outcome measures and appropriate statistical tests
• Summary of adverse events by study group
• Administrative information about the study and a description of any agreements that restrict the disclosure of study results

Although reporting all results—including negative results—is critical to the scientific process, overall compliance with the requirement to submit results to ClinicalTrials.gov is poor (Anderson et al 2015; Califf et al 2025; DeVito et al 2020; Miller et al 2015; Piller et al 2015; Riley et al 2018; Zarin et al 2019). Researchers have highlighted an ongoing need for strategies to facilitate high-quality submissions to ClinicalTrials.gov, reduce reporting burden, improve incentives to encourage reporting, and highlight the value of reporting as an integral part of the research process (Zarin et al 2019).

The Clinical Trials Transformation Initiative (CTTI) conducted in-depth interviews with researchers, administrators, and others to identify factors that lead to late registration and incomplete or missing results information for studies registered in ClinicalTrials.gov (CTTI 2024). When asked to describe challenges with trial registration and reporting, survey respondents cited lack of understanding on the part of the responsible parties about the requirements and what information must be submitted; nonresponsive study teams; and concerns on the part of study teams about submitting results information to ClinicalTrials.gov before all data are analyzed and published.

Respondents to the CTTI survey agreed that improving the availability of resources to educate responsible parties about ClinicalTrials.gov requirements would help with the timeliness and completeness of registration and reporting. Such resources could include training and tip sheets, a summary of common mistakes during the submission process, training videos, in-person and virtual training conferences and workshops, and better communication about guidance and compliance actions. Trial sponsors also should plan for providing adequate resources to cover the time and effort to update the results.

Essential existing resources include:

• ClinicalTrials.gov Protocol Registration and Results System Help Resources
• ClinicalTrials.gov Support and Training Materials

In addition, CTTI created the Aggregate Analysis of ClinicalTrial.gov (AACT) database to enable researchers to easily access and analyze data submitted to ClinicalTrials.gov. The database, which is refreshed daily, contains all information about every study registered in ClinicalTrials.gov.