Objective: To investigate the diagnostic reliability of mandibular second molar maturation in assessing the mandibular growth peak using a longitudinal design. Materials and Methods: From the files of the Burlington and Oregon growth studies, 40 subjects (20 from each collection, 20 males and 20 females) with at least seven annual lateral cephalograms taken from 9 to 16 years were included. Mandibular second molar maturation was assessed according to Demirjian et al., and mandibular growth was defined as annual increments of Co-Gn distance. A full diagnostic reliability analysis (including positive likelihood ratio) was performed to establish the diagnostic reliability of dental stages E, F, and (pooled) GH in identifying the imminent mandibular growth peak. Results: None of the dental maturation stages reliably identified the mandibular growth peak with greatest overall mean accuracy and positive likelihood ratio of 0.77 (stage F) and 2.7 (stage E), respectively. Conclusions: Use of the mandibular second molar maturation is not recommended for planning treatment requiring identification of the mandibular growth peak. (Angle Orthod. 2017;87:665-671.)