Aim: To compare and evaluate long-term stability of crossbite correction with Quad Helix or expansion plate in the mixed dentition. Methods: In this RCT-study 35 patients with unilateral posterior crossbite were randomized to be treated with either Quad Helix or expansion plate. The inclusion criteria were: mixed dentition, unilateral posterior crossbite, no sucking habits or previous orthodontic treatment. Stability was evaluated after 3 years by study cast measurements. Twenty subjects with normal occlusion were included as controls. Success rate, maxillary and mandibular transverse dimensions, overjet and overbite were registered. Results: Stability was equal for the two treatment methods. Small, albeit significant, differences between the groups were assessed with reference to transverse dimensions. No significant difference was seen for overjet and overbite. Conclusions: The long-term stability of posterior crossbite correction with Quad helix and expansion plate was equal.