The edentulous ridge expansion (ERE) technique is a partial-thickness flap procedure for implant surgery that was developed in 1986. The ERE technique permits the use of osseointegrated implants at sites whose minimal orofacial dimensions are insufficient for traditional implant surgery. The present work is a histologic and ultrastructural study of hard tissue repair modalities at edentulous sites that were treated with the ERE technique in 20 humans. Biopsies were obtained from the tissue regenerating within the surgically created bone gap between the mobile buccal and nonmobile lingual or palatal bone-periosteum plates ('bone flaps') on days 40, 90, 120, 150, and 480 (day 0 = day of implant insertion). The results suggest that osteoblasts differentiate from preexisting mesenchymal cells located on the original fissure walls, with consequent deposition of new bone in the surgically created intrabony defect.