The purpose of this study was to evaluate treatment outcomes and long-term stability in patients treated by surgically assisted rapid maxillary expansion (SARME) and to compare the results with a matched, untreated control group. The sample comprised consecutive study models from 31 subjects (17 males and 14 females) with a mean pre-treatment age of 25.9 years [standard deviation (SD) 9.6]. The mean follow-up time was 6.4 years (SD 3.3). The transverse distances between the maxillary canines and maxillary first molars were measured with digital sliding callipers before treatment (T0), after treatment (T1), and at follow-up (T2). The data were analysed with a Mann-Whitney U, Spearman's rho, and Wilcoxon signed-rank tests. At T1, all posterior crossbites were corrected and the expansions were statistically significant. At T2, despite some reduction in the transverse measurements, the posterior crossbites remained corrected. There were no statistically significant differences between the treatment and control groups at T2 regarding transverse measurements, except for the distance between the mesio-buccal cusp tips of the maxillary first molars. In the treatment group, there was no significant difference in terms of reductions in the transverse dimensions over the short- or long-term, no significant correlations between age or gender and the decrease in transverse dimensions or between the degree of anterior and posterior expansion. There were no significant correlations between the degree of expansion and subsequent post-treatment decrease. The results indicate that SARME normalizes the transverse discrepancies and is stable a mean of 6 years post-treatment. The decreases in the transverse dimensions are most pronounced during the first 3 years post-treatment.