This epidemiological survey was conducted on 2,016 children from 8 private and 12 public preschools at the city of Bauru, São Paulo, Brazil. The sample was composed of 1,032 males and 984 females in the primary dentition stage, aged 3 to 6 years. Normal occlusion was observed in 26.74% of the sample; thus, 73.26% of children presented some type of malocclusion. Among the malocclusions, the following transverse problems were diagnosed: unilateral posterior crossbite (11.65%), anterior open bite associated with posterior crossbite (6.99%), bilateral posterior crossbite (1.19%), unilateral posterior crossbite associate with anterior crossbite (0.79%) and full crossbite (0.19%) totalizing 20.81% of the transverse problems. Mandibular functional deviation was observed in 91.91% of children with unilateral posterior crossbite, characterizing the functional unilateral posterior crossbite. The results demonstrated that the prevalence of posterior crossbite was compatible with previous data in the literature, with predominance of functional unilateral posterior crossbite.