OBJECTIVE: To examine compliance with mandatory reporting of summary clinical trial results (within one year of completion of trial) on ClinicalTrials.gov for studies that fall under the recent Food and Drug Administration Amendments Act (FDAAA) legislation. DESIGN: Registry based study of clinical trial summaries. DATA SOURCES: ClinicalTrials.gov, searched on 19 January 2011, with cross referencing with Drugs@FDA to determine for which trials mandatory reporting was required within one year. SELECTION CRITERIA: Studies registered on ClinicalTrials.gov with US sites which completed between 1 January and 31 December 2009. MAIN OUTCOME MEASURE: Proportion of trials for which results had been reported. RESULTS: The ClinicalTrials.gov registry contained 83,579 entries for interventional trials, of which 5642 were completed within the timescale of interest. We identified trials as falling within the mandatory reporting rules if they were covered by the FDAAA (trials of a drug, device, or biological agent, which have at least one US site, and are of phase II or later) and if they investigated a drug that already had approval from the Food and Drug Administration. Of these, 163/738 (22%) had reported results within one year of completion of the trial compared with 76/727 (10%) trials that were not subject to mandatory reporting (95% confidence interval for the difference in proportions 7.8% to 15.5%; χ2 test, P=2.6x10^-9). Later phase trials were more likely to report results (P=4.4x 10^-11), as were industry funded trials (P=2.2 x10^-16). CONCLUSION: Most trials subject to mandatory reporting did not report results within a year of completion.