To assess evidence from randomised clinical trials about the effectiveness of extracts of Boswellia serrata (frankincense).Systematic review.Electronic searches on Medline, Embase, Cinahl, Amed, and Cochrane Library. Hand searches of conference proceedings, bibliographies, and departmental files.All randomised clinical trials of B serrata extract as a treatment for any human medical condition were included and studies of B serrata preparations combined with other ingredients were excluded. Titles and abstracts of all retrieved articles were read and hard copies of all relevant articles were obtained. Selection of studies, data extraction and validation were done by the author. The Jadad score was used to evaluate the methodological quality of all included trials.Of 47 potentially relevant studies, seven met all inclusion criteria (five placebo controlled, two with active controls). The included trials related to asthma, rheumatoid arthritis, Crohn's disease, osteoarthritis, and collagenous colitis. Results of all trials indicated that B serrata extracts were clinically effective. Three studies were of good methodological quality. No serious safety issues were noted.The evidence for the effectiveness of B serrata extracts is encouraging but not compelling.