Abstract A scoring system for registration of apical periodontitis in radiographs is presented. The system is termed the periapical index (PAI) and provides an ordinal scale of 5 scores ranging from 1 (healthy) to 5 (severe periodontitis with exacerbating features). Its validity is based on the use of reference radiographs of teeth with verified histological diagnoses. Results from studies involving 11 observers and 47 selected radiographs document that the PAI system is reasonably accurate, reproducible and able to discriminate between sub-populations. It may also allow for results from different researchers to be compared. The system may be suitable for the analysis of periapical radiographs in epidemiological studies, in clinical trials and in retrospective analyses of treatment results in endodontics.