OBJECTIVE To evaluate short- and long-term maxillary dental arch dimensional changes in patients treated with a transpalatal arch (TPA) during mixed dentition followed by full fixed appliances in the permanent dentition compared with an untreated sample. METHODS Dental casts and lateral cephalograms obtained from 36 consecutively treated patients before TPA treatment (T0), after TPA treatment (T1), after fixed appliance treatment (T2), and a minimum of 3 years after fixed appliance treatment (T3) were analyzed. The control group was matched as closely as possible. Arch widths, perimeter, and length, as well as crowding and incisor proclination, were evaluated. RESULTS In the treated group, intercanine, interpremolar, and intermolar widths and arch perimeter increased significantly at T1. At T2, only the intercanine width increase was still significant. At T3 all arch dimensions decreased, remaining larger than they were at T0. The arch length increased after T1, significantly decreased at T2, and slightly decreased at T3. The crowding decreased significantly at T1, was eliminated at T2, and increased at T3. At T3, 50% of the patients showed relapse with crowding ranging from 0.5 to 2 mm. In the control group at T1, only slight changes were noted but crowding increased. At T2, crowding and upper incisor inclination increased but arch length decreased. At T3, intercuspid width, arch perimeter, and arch length continued to decrease, thereby increasing crowding. CONCLUSIONS Maxillary dental arch dimensions changed significantly after TPA followed by treatment with fixed appliances. Relapse occurred to some extent, especially in intercanine width and arch perimeter, but most of the dental arch changes remained stable.