This study reports on the pattern of interradicular bone formation and the frequency and localization of ankylosis following regenerative surgery of experimentally-induced furcation defects in 12 beagle dogs. Observations were made in 39 specimens showing healing with connective tissue attachment in the full circumference of the furcation. The findings indicate that bone regeneration first occurs adjacent to the root surfaces, followed by a subsequent fill of the central portion of the interradicular area. Ankylosis, observed in 24 of the 39 specimens, was more frequent in large defects than in small defects. The incidence of ankylosis increased with increased distance from the base of the original wound. These findings suggest that coronal bone apposition may require or be facilitated by the presence of a root surface, and that undisturbed coronal regeneration may be dependent upon cellular or some other activity from the intact periodontal ligament.