The purpose of this cross-sectional study was to determine whether patients exhibiting third molar agenesis also exhibit delayed development and/or eruption of the adjacent second molar. The initial panoramic radiographs of 739 patients, 13-17 years of age, from the University of Iowa's Department of Orthodontics were examined. The developmental stage of each second molar according to Demirjian's 8-stage classification system, the eruption stage of each second molar, along with the presence or absence of each third molar were recorded, and Pearson chi-squared tests were used to determine differences in developmental and eruptive timing between those missing third molars and those with third molars present. We conclude that patients missing their maxillary third molars exhibit delayed development of the adjacent maxillary second molars. In addition, patients missing any of their four third molars exhibit delayed eruption of their adjacent second molars. The findings also suggest that mandibular second molars develop later than, but erupt earlier than, maxillary second molars.