The purpose of this study was to estimate the diagnostic availability of the common periapical lesions by using computer. The author used a domestic personal computer and rearranged the applied program appropriately with RF (Rapid File), a program to answer the purpose of this study, and then input the consequence made out through collection, analysis and classification of the clinical and radiological features about the common periapical lesions as a basic data. The 256 cases (Cyst 91, Periapical granuloma 74, Periapical abscess 91) were obtained from the chart recordings and radiographs of the patients diagnosed or treated under the common periapical lesions during the past 8 years (1983-1990) at the infirmary of Dental School, Chosun University. Next, the clinical and radiographic features of the 256 cases were applied to RF program for diagnosis, and the diagnosis by using computer was compared with the hidden final diagnosis by clinical and histopathological examination. The obtained results were as follows: 1. In cases of the cyst, diagnosis through the computer program was shown rather lower accuracy (80.22%) as compared with accuracy (90.1 %) by the radiologists. In cases of the granuloma, diagnosis through the computer program was shown rather higher accuracy (75.7%) as compared with the accuracy (70.3%) by the radiologists. 2. In cases of periapical abscess, the diagnostic accuracy was shown 88% in both diagnoses. 4. The average diagnostic accuracy of 256 cases through the computer program was shown rather lower accuracy (81.2%) as compared with the accuracy (82.8%) by the radiologists. 5. The applied basic data for radiographic diagnosis of common periapical lesions by using computer was estimated to be available.