The aim of this work was to evaluate the eruption of the third molar teeth in the length of the mandible using the orthodontic documentation obtained between 2005 and 2006 as sampling method, a total of 193 cases of both sexes. The cephalometric analysis and panoramic radiographic studies were chosen from two clinics randomly selected in the city of Recife - Pernambuco - Brazil. It was analyzed the linear size of mandibular bone by the use of McNamara's Cephalometric Analysis, and also the presence or not of dental retention of inferior third molar teeth with the help of panoramic radiographic study, evaluating the commitment of these retentions with the effective length of mandible. The work concluded that when the effective mandibular length is small induces a dental retention of these third molars, mostly the left inferior third molar, which 46.5% of the cases was observed as retained. However, in 53.8% of the cases was observed the absence or no-formation of these dental germs for the left inferior third molar and 60% for the right, showing a big size of the effective length of the mandible exposing that the no-formation of these teeth was not directly related to the lack of space in the dental arch. The study demonstrated that the sexual dimorphism does not influence the eruption of these teeth, however, according to age, was emphasized a dependence concerning about dental eruption, retention or absence of left inferior third molar.