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Objective: To evaluate the long-term effects of successful slow maxillary expansion without fixed appliances or retainers in the mixed dentition on patients with unilateral crossbites, using Haastype, hyrax, or quad helix appliances. Materials and Methods: Serial dental casts of 110 patients were evaluated at three time points: preexpansion (T1) (mean age 7 years/7 months), postexpansion (T2) (mean age 8 years/ 8 months), and approximately 4 years later in the permanent dentition (T3) (mean age 12 years/ 9 months). Maxillary and mandibular intercanine and intermolar widths, arch length, and perimeter and molar angulation were measured at all three time intervals with the Michigan published growth norms serving as a control. Results: Successful treatment by slow maxillary expansion (SME) produced similarly favorable expansion by all three expanders in all measurements for both arches. Maxillary arch widths were narrower than controls pretreatment (T1) and wider than controls immediately post treatment (T2). Long-term (T3) maxillary intermolar width was the same as controls, with intercanine width significantly wider than controls. Maxillary intercanine and intermolar width increased from T1 to T3, by 4.5 mm and 3.5 mm, respectively, with 98% of intercanine and 80% of intermolar expansion remaining at T3. Maxillary arch circumference increased by 1 mm from T1 to T3. Mandibular width did not change significantly. Conclusion: Maxillary arch dimensions in early mixed dentition in patients with unilateral posterior crossbite showed good stability 4 years post treatment in the permanent dentition. (Angle Orthod. 2011;81:616-623.)