Management of retained deciduous second molars in the absence of permanent successors can be challenging. The purpose of this retrospective radiographic study was to examine long-term changes in retained deciduous mandibular second molars in adult patients. Specifically, we sought to answer the question, "If an adult has a retained deciduous mandibular second molar and a missing successor, is retaining the deciduous molar a viable treatment option?" From more than 6000 adult patients at the University of Iowa College of Dentistry, 20 were identified who initially had at least 1 retained deciduous mandibular second molar without a permanent successor and who had radiographs spanning 5 or more years after the initial visit. The mean (standard deviation) age at the initial examination was 36.1 (12.9) years, the mean age at the final examination was 48.5 (12.6) years, and the mean examination time span was 12.4 (7.7) years. Of the 28 retained deciduous molars initially identified, 24 (86%) continue to function. The remaining 4 were lost at a mean age of 51.0 (13.9) years and a mean time span after initial examination of 14.3 (10.0) years; this approximates the lifespan of some prosthetic appliances. These 4 mandibular deciduous molars were lost because of caries or periodontal breakdown. Furthermore, we found that the average shortening of all deciduous root lengths was negligible (0.16 mm), and only 6 teeth underwent a change in restoration status. Based on this study in adults and earlier studies in adolescents, we conclude that retaining healthy deciduous mandibular second molars is a viable treatment alternative.