Objective To identify and compare studies quantifying inter-canine width pre- and post-treatment and to propose a baseline for canine adjustment.Data sources A search of Medline and unpublished master's thesis and references cited in located articles. Hand searching of recent issues of relevant journals was also carried out. Authors were contacted for access to mean or primary data.Study selection Studies were included if numerical data was available to show changes in inter-canine width from pre to post treatment and post retention, if there was adequate information relating to the subjects, treatment retention devices and post retention records. When more than one study used the same patient records only the original paper was used. 26 studies met the criteria.Data extraction and synthesis Data from 1233 patients was pooled to give an overall mean change in inter-canine width. Subgroup analysis was also performed based on Angles Incisor classification and extraction and non-extraction therapy.Results The table shows the changes in inter-canine width in millimetres Conclusions Mandibular inter-canine width tends to expand during treatment by 0.8–2.0 mm, and constrict post-retention by 1.2–1.9 mm regardless of pre-treatment classification or whether treatment was extraction or non-extraction. Post-retention mandibular inter-canine widthshows a net change from 0.5 mm expansion to 0.6 mm constriction. Since the net change is approximately zero this study supports the concept of maintenance of original inter-canine width in orthodontic treatment.