Background: We evaluated empirically whether trial registries provide useful information to evaluate the quality of randomized clinical trials (RCTs). Methods: We compared the report of methodological characteristics from a random sample of ongoing RCTs registered in six World Health Organization (WHO) Primary Registries and ClinicalTrials.gov in 2008. As 90% of trials were provided from ClinicalTrials.gov, we ensured adequate representation across registries by including a representative sample of each registry. We assessed the reporting of relevant domains from the Cochrane Collaboration’s ‘risk of bias’ tool and other key methodological aspects. Two reviewers independently assessed each record. Results: A random sample of records of actively recruiting RCTs was retrieved from seven registries using the WHO ICTRP search portal. Overall, 101 RCTs were open (38%), 141 blind (53%) and in 23 records no sufficient information was provided for judging. Weighted overall proportions in the ICTRP search portal for adequate reporting of sequence generation, allocation concealment and blinding (including and excluding open label RCT) were 5.23% (95% CI 2.55 to 7.91), 1.37% (95% CI 0.0 to 2.76%), 41.1% (95% CI 35.08 to 46.92) and 8.37% (95% CI 4.08 to 12.52) respectively. Most items had insufficient or no information to permit judgment. Significant differences in the proportion of adequately reported RCTs were found between registries that had specific methodological fields for describing methods of randomization and allocation concealment compared to registries that did not (random sequence generation, 74% vs. 2%, p < 0.001; allocation concealment, 53% vs. 0%, p < 0.001). Concerning other key methodological aspects, weighted overall proportions of RCTs with adequately reported items were as follows: eligibility criteria (81%), primary outcomes (66%), secondary outcomes (46%), follow-up duration (62%), description of the interventions (53%) and sample size calculation (1%). Conclusion: Registries with specific methodological fields obtained more relevant quality information than those with general or coded fields. Critical appraisal of RCTs should include a search for information on trial registries as a complement to journal publications. However, the usefulness of the information will vary across registries due to variable content.