Background: Records for more than 100,000 clinical trials are publicly available on registries that meet international requirements for data quality and transparency. The extent to which systematic reviewers are using these registries to identify potentially eligible studies is unknown. Objectives: To assess if and how Cochrane reviewers are searching databases of ongoing studies. Methods: All new protocols and reviews published in Issue 2, 2010 of The Cochrane Database of Systematic Reviews were eligible except overviews, diagnostic accuracy reviews and methodology reviews. Two authors independently evaluated the search strategy of each protocol and review and extracted data on the search of databases of ongoing studies. Disagreements were resolved by consensus. Results: 41 protocols and 26 reviews were included. A search of a database of ongoing trials was mentioned in 25 (61%) protocols and 10 (38%) reviews (Table 1). Of these 35 documents, only 1 review and 1 protocol specified the terms for this search. Searches for ongoing studies are likely to be inefficient (eg searching the same data through different web sites) and inadequate (eg limited to a single database). Review authors often conduct high effort/low yield tasks such as handsearching conference proceedings or contacting experts, but do not search structured databases of trials (low effort/potentially high yield). There are also problems with accuracy (eg listing databases to be searched that are not yet publicly available) and clarity (eg imprecise names such as ‘‘the NIH database’’). Conclusions: Search strategies for ongoing or prospectively registered trials in Cochrane protocols and reviews are poorly documented. There is confusion about the various databases and registers that exist and the overlap between them. The WHO ICTRP database, which includes records from most of the databases mentioned by review authors, is being underused. Suggestions for clearer guidance will be proposed.