Three cases of secondary retention of the first permanent molar and impaction of the adjacent second molar are reported. The clinical, radiographic, and histologic characteristics are described. In all three cases, the retained molars were in severe infra-occlusion and they may have represented a physical barrier for the impacted second molar. Early recognition of secondary retention is of great importance, because treatment of this disorder in children and adolescents is relatively simple. If secondary retention of permanent molars is not recognized before adulthood, severe malocclusion may develop, and removal of the afffected molar will be more difficult.