An article in PLoS One, by researchers at the National Institute of Health in Bethesda, Maryland, finds that less than half (45%) of clinical trials published any evidence of results. Less than 10% reported results through both clinical trial registration and publication.
Huser, V., Cimino, J.; Linking ClinicalTrials.gov and PubMed to Track Results of Interventional Human Clinical Trials. PLoS One. Online July 9, 2013