Objective: The purpose of this study was to determine and compare the incidence, location, and size of accessory foramina in the furcation region of permanent and primary molars. Method and materials: A random sample of 100 extracted human permanent maxillary and mandibular first and second molars (25 teeth of each type) and a random sample of 100 extracted human primary maxillary and mandibular molars (50 teeth of each type) were used. The crowns and roots of each tooth were removed at a point 1.5 mm apical to the external furcation region, and a second cut was made at a point 1 mm apical to the cementoenamel junction. The specimens were examined using scanning electron microscopy at magnifications ranging from 310 to 31,250. The incidence, location, and size of accessory foramina were documented and statistically analyzed. Results: Of the 100 permanent molars examined, 79% had accessory foramina with diameters ranging from 10 µm to 200 µm. Accessory foramina were present in 94% of the primary molars, with diameters varying from 10 µm to 360 µm. The incidence of accessory foramina was significantly higher in primary than in permanent molars. Conclusion: The presence of accessory foramina with large diameters may imply that an inflammatory process can spread from pulpal to periodontal tissues and vice versa.