Background The initial arch wire is the first arch wire to be inserted into the fixed appliance at the beginning of orthodontic treatment and is used mainly for correcting crowding and rotations of teeth. With a number of orthodontic arch wires available for initial tooth alignment, it is important to understand which wire is most efficient, as well as which wires cause the least amount of root resorption and pain during the initial aligning stage of treatment. Objectives To identify and assess the evidence for the effects of initial arch wires for alignment of teeth with fixed orthodontic braces in relation to alignment speed, root resorption and pain intensity. Search strategy We searched the following electronic databases: the Cochrane Oral Health Group's Trials Register (30th November 2009), CENTRAL (The Cochrane Library 2009, Issue 4), MEDLINE (1950 to 30th November 2009) and EMBASE (1980 to 30th November 2009). Reference lists of articles were also searched. There was no restriction with regard to publication status or language of publication. We contacted all authors of included studies to identify additional studies. Selection criteria Randomised controlled trials (RCTs) of initial arch wires to align crooked teeth with fixed orthodontic braces were selected. Only studies involving patients with upper and/or lower full arch fixed orthodontic appliances were included. Data collection and analysis Two review authors were responsible for study selection, validity assessment and data extraction. All disagreements were resolved by discussion amongst the review team. Corresponding authors of included studies were contacted to obtain missing information. Main results Seven RCTs, with 517 participants, provided data for this review. Among them, five trials investigated the speed of initial tooth alignment comparing: 0.016 inch ion-implanted A-NiTi wire versus 0.016 inch A-NiTi versus 0.0175 multistrand stainless steel wire; 0.016x0.022 inch medium force active M-NiTi wire versus 0.016x0.022 inch graded force active M-NiTi wire versus 0.0155 inch multistrand stainless steel wire; 0.016 inch superelastic NiTi wire versus 0.016 inch NiTi wire; 0.014 inch superelastic NiTi wire versus 0.0155 inch multistrand stainless steel wire; 0.016 inch CuNiTi wire versus 0.016 inch NiTi wire. The other two studies investigated pain intensity experienced by patients during the initial stage of treatment comparing: 0.014 inch superelastic NiTi wire versus 0.014 inch NiTi wire; 0.014 inch superelastic NiTi wire versus 0.015 inch multistrand stainless steel wire. Data analyses were often inappropriate within the included studies. Authors' conclusions There is some evidence to suggest that there is no difference between the speed of tooth alignment or pain experienced by patients when using one initial aligning arch wire over another. However, in view of the general poor quality of the including trials, these results should be viewed with caution. Further RCTs are required.