The present study aimed to evaluate the need for orthodontic treatment in Brazilian schoolchildren presenting both late mixed dentition and early permanent dentition as well as to determine the possible factors associated with this necessity. Our randomly selected sample consisted of 407 schoolchildren aged between 9 and 12 years from Nova Friburgo (State of Rio de Janeiro), Brazil. All the children were evaluated according to the two components of the Index of Orthodontic Treatment Need (IOTN), namely, the Dental Health Component (DHC) and the Aesthetic Component (AC). Data analysis involved multiple logistic regression. Definite need for orthodontic treatment was found in 34.2% and 11.3% of the children according to, respectively, DHC and AC. The most prevalent malocclusions included the following: contact point displacement (crowding), crossbite, and increased overjet. Male gender, permanent dentition and aesthetic reasons were factors associated with a great need for orthodontic treatment. It was concluded that about one-third of the children evaluated had a definite need for orthodontic treatment. This necessity was greater in the permanent dentition, thus emphasizing the importance of an early identification of malocclusions and a timely referral of patients for treatment. The correlation between AC and DHC was considered important since they involve distinctive characteristics.