OBJECTIVE To evaluate the transverse relationships of the first permanent molars after the correction of posterior crossbite performed during the deciduous dentition with two different treatment protocols. METHODS Ninety patients (40 males and 50 females) with posterior crossbite were treated during the deciduous dentition with either a removable expansion plate (RP group, 60 patients) or rapid maxillary expander (RME group, 30 patients). The dental casts before treatment (T0, mean age 5.1 ± 0.7 years) and after treatment, when the first permanent molars were fully erupted, (T1, mean age 7.7 ± 1.0 years) were analysed. The prevalence rates for posterior crossbite on the first permanent molars in the two groups were compared by means of chi-squared test with Yates correction. A logistic regression was performed to evaluate the factors that could influence the presence/absence of posterior crossbite on the first permanent molars. RESULTS Prevalence rate of posterior crossbite on the first permanent molars at T1 was 34.4 per cent in the total sample, 28.3 per cent for the RP group and 46.6 per cent for the RME group. No significant predictors for the 'presence/absence of posterior crossbite on the first permanent molars' at T1 were found. CONCLUSIONS Retrospective study. CONCLUSIONS After treatment of posterior crossbite during the deciduous dentition phase, the first permanent molars erupted in crossbite in the 34.4 per cent of the cases. The type of treatment is not a significant predictor for the presence of posterior crossbite on the first permanent molars.