This preliminary report describes our evaluation of the behavior of permanent maxillary first molars subsequent to rapid maxillary expansion (RME) in the mixed dentition in crossbite and non-crossbite cases; maxillary deciduous canines and second molars were used as anchorage. The RME appliance was cemented on the maxillary deciduous second molars and canines in 19 (13 females, six males) patients with unilateral or bilateral permanent molar crossbites (Group A) and in 13 (10 females, three males) patients who had no crossbites (Group B). After a mean appliance activation period of 15 days for 4-8 mm expansion, the appliance was stabilized and kept in place as retention for 5-10 months. Neither active nor passive retention was applied on first permanent molars. Impressions of dental arches were made before insertion of the appliance (T1), at stabilization (T2), and at debanding (T3). Dental measurements were registered at T1, T2, and T3. In Group A, the mean maxillary permanent intermolar width was 40.6 mm at T1, 46.0 mm at T2, and 46.3 mm at T3. In Group B, 42.4 mm at T1, 46.3 mm at T2, and 45.4 mm at T3. In all subjects, the intermolar width increased during the active phase, and in Group A the crossbite was corrected and the correction was spontaneously stable. In these patients, deciduous teeth could support the heavy forces produced by the RME appliance. The maxillary intermolar distance was spontaneously increased and then maintained with no retention in Group A. The mean spontaneous expansion of permanent maxillary molars was approximately half of the mean expansion of the screw and the anchor deciduous teeth.