'Posterior crossbite' occurs when the top back teeth bite inside the bottom back teeth. When it affects one side of the mouth the lower jaw may have to move to one side to allow the back teeth to meet together. This movement may have long term effects on the growth of the teeth and jaws. It is unclear what causes posterior crossbites and they may develop or improve at any time from when the baby teeth come into the mouth to when the adult teeth come through. Several treatments have been recommended to correct them. Some treatments widen the upper teeth whilst others are directed at treating the cause of the posterior crossbite e.g. breathing problems or sucking habits. Most treatments have been used at each stage of dental development.The aim of this review was to identify and evaluate orthodontic treatments used to expand the maxillary dentition and / or correct posterior crossbites.All randomised and controlled clinical trials identified from the Cochrane Controlled Trials Register according to the Oral Health Group Search Strategy and stored in the Cochrane Collaboration Oral Health Group Database of Clinical Trials, a MEDLINE search using the Mesh term and free text words, hand searching the British, European and American journals of orthodontics and Angle Orthodontist, and the bibliographies of papers and review articles which reported the outcome of orthodontic treatment to expand the maxillary dentition and/or correct a posterior crossbite that were published as abstracts or papers between 1970 and 1997 in English.All randomised and controlled clinical trials published as full papers or abstracts which reported quantitative data on the outcomes crossbite correction, molar and/or canine expansion, signs and symptoms of temporomandibular joint dysfunction or respiratory disease.Data were extracted without blinding to the authors, treatments used or results obtained. The first named authors of randomised and controlled clinical trials were written to in an attempt to establish the method of randomisation / allocation and identify unpublished studies. Odds ratio, 95% confidence intervals, relative risk, relative risk reduction, absolute risk reduction, and the number need to treat were calculated for event data. The weighted mean difference and 95% confidence intervals were calculated for continuous data.Using the search strategy 7 randomised and 5 controlled clinical trials were identified but following correspondence with the authors, 3 of the randomised and 1 of the controlled clinical trials were reclassified giving 5 randomised and 7 controlled clinical trials for inclusion in the review. Trials comparing occlusal grinding in the primary dentition with/without an upper removable expansion appliance in the mixed dentition versus no treatment, banded versus bonded rapid maxillary expansion, banded versus bonded slow maxillary expansion, transpalatal arch with/without buccal root torque and an upper removable expansion appliance versus quad-helix were identified. Occlusal grinding in the primary dentition with/without the addition of an upper removable expansion plate, in the mixed dentition for those children who did not respond to grinding, was shown to be effective in preventing a posterior crossbite in the primary dentition from being perpetuated to the mixed and permanent dentitions. No evidence of a difference in treatment effect (molar and canine expansion) between the test and control intervention was found in the trials which compared banded versus bonded rapid maxillary expansion, banded versus bonded slow maxillary expansion, transpalatal arch with/without buccal root torque, or upper removable expansion appliance versus quad-helix. (ABSTRACT TRUNCATED)