Results reported only in the form of conference abstracts are often included in clinical systematic reviews. Some literature sources such as Cochrane Central include conference abstracts as part of their database, while hand-searching of relevant conferences may be partaken as part of the review process. OBJECTIVE: The objective of this research was to quantify the contribution of conference abstracts to high-quality clinical systematic reviews, and investigate the use of hand-searching in addition to database searching for the identification of relevant abstracts. METHODS: Reviews were included if they examined clinical outcomes of a pharmaceutical intervention. All suitable reviews in Issues 3 and 4, 2008, of the Cochrane Database of Systematic Reviews were used. In addition, we searched the British Medical Journal between 2000 and 2008 for suitable systematic reviews by using the search term “systematic review” in the title; the first 20 which fulfilled the study criteria were included. Further, we examined three large clinical systematic reviews produced in-house between 2007 and 2008. RESULTS: Thirty-seven systematic reviews were examined from the Cochrane Database. A total of 26 reviews included no conference abstracts. Of 567 included studies in all reviews, 25 (4%) were reported as conference abstracts only. Among the six reviews in oncology, 10% (17/173) of the included studies were conference abstracts. Twenty out of 370 studies across 20 systematic reviews published in the BMJ were conference abstracts only; among the 10 reviews including hand-searching of conference proceedings, this was 7%, compared to 4% for those not. Of the 664 studies found in in-house reviews, 79 studies were only reported in conference abstracts (12%), of which 44 were found only through hand-searching. CONCLUSION: Conference abstracts form a small proportion of included studies in high-quality clinical systematic reviews. Hand-searching of conference proceedings makes a modest contribution to the number of included studies.