Background: the number of authors per publication has increased in many biomedical journals.

Objectives: this study investigated the development in the number of authors per Cochrane Review from 1996 to 2018.

Methods: we studied a total of 7447 Cochrane Reviews, based on data from the Cochrane Database of Systematic Reviews. We registered the numbers of authors per Cochrane Review and within each of the 55 editorial groups. We examined the 'Contribution of authors' section individually for 27 Cochrane Reviews with more than 15 authors.

Results: the median number of authors per Cochrane Review was three in 1996 (range 2 to 3), four (range 1 to 13) in 2007, and five (range 2 to 22) in 2018. A total of 35 editorial groups showed increasing trends, 20 editorial groups revealed no changes, and none of the editorial groups showed decreasing trends in the number of authors. According to the reporting in the 'Contribution of authors' section for the Cochrane Reviews with more than 15 authors, not all authorships could be justified.

Conclusions: the number of authors per Cochrane Review has gradually increased from 1996 to 2018. This increase could reflect that preparing a Cochrane Review has increased in complexity or reflect the presence of possible 'gift authorships'. A fraction of multi-author reviews may include gift authors.

Patient or healthcare consumer involvement: not relevant