Biopsy reports from 150 periradicular tissue specimens obtained from teeth refractory to nonsurgical endodontic therapy were reviewed. The specimens were submitted by postdoctoral dental students in the Department of Endodontics, and the biopsy reports were prepared by oral pathologists at the University of Texas Health Science Center at San Antonio. The study found that 59.3% of the periradicular lesions were granulomas, 22% cysts, 12% scars, and 6.7% other pathoses. The majority (56%) of endodontically treated cases which failed to heal were recognized within 2 yr after the completion of therapy. The most common location for surgical retreatment was the anterior maxilla, followed by the posterior maxilla, the posterior mandible, and the anterior mandible. The periapical granuloma was the predominant pathosis at each location.