Objective: To determine the longest upright and tilted implant that can be placed in posterior mandibular teeth (second premolars [2PM], first molars [1M], and second molars [2M]) in case of immediate implant placement (IIP), respecting a safe distance from the inferior alveolar canal (IAC) and the submandibular fossa, using cone beam computed tomography (CBCT) scans. Method and Materials: In CBCT scans of 118 subjects, the following were measured: distance from the tooth apex to the IAC (DTC), lateromedial width of the mandible, classification of mandibular cross-sectional morphology, upright and tilted implant placement simulation, tilting angle that allowed placement of the longest implant. Results: The DTC has shown to be greater for 1M, shorter for 2PM, and even shorter for 2M. Between 70% and 88% of all 2PM, 1M, and 2M present less than 6 mm of DTC. The cross-sectional morphology of the mandible had a tendency to be classified as "undercut" in more posterior regions. For both 4.0- and 5.0-mm implants, more posterior regions resulted in a greater difference in length between the upright and tilted implants. The tilting angle was greater for the more posterior regions. Most of the relationships between age/sex and the morphometric parameters were weak. Conclusion: Lingual concavity is a common clinical finding at the posterior mandibular region, and its occurrence varies according to the tooth type. The great majority of the 2PM, 1M, and 2M present limited available bone between the root apex and the IAC, which stresses the need for careful attention when IIP is planned in this area.