Research in education has demonstrated the usefulness of verbal mediators as memory aides. The mediator or mnemonic POOR PAST can be of use to the dentist in recalling the list of alternatives in the differential diagnos of endodontic failures: P--perforation; O--obturation; O--overfill; R--root canal missed; P--periodontal disease; A--another tooth; S--split; T--trauma. With a comprehensive list of diagnoses, recall of the appropriate questions and tests in examining for each possibility is simplified. The diagnostician then proceeds to look for a specific symptom or symptoms that would confirm a particular diagnosis or that would limit the alternatives to two or three possibilities. Questioning about the nature and duration of the symptoms is useful in ruling out some diagnoses. Pulp tests, periodontal probing, radiographs from different angles, percussion, palpation, and sinus tract exploration each have special application to one or more of the possibilities on the list. The wise clinician also evaluates access preparations, occlusion, and those etiologic factors that could cause pulp or periodontal disease. Variations in the root canal, requirements of treatment, the nature of the healing process, expected results, and surgical principles are considered along with the foregoing in problem solving. After a case has been thoroughly investigated and if the POOR PAST alternatives have been ruled out the dentist may then conclude that for some as yet undetermined reason the lesion exceeds the recuperative powers of the host and that further consultation or surgical intervention will be necessary.