The application of evidence-based dentistry to diagnosis should result in a reduction of errors in decision making. The frequency of errors is dependent not only on the accuracy of a diagnostic test for pathology, but also on the prior chance of disease being present. If this chance is low and below a certain threshold, then, for example, applying a diagnostic test can result in more decision errors and therefore inappropriate treatment than omitting to use the test. In deciding on the usefulness of a diagnostic test an additional factor to take into account is the relative value of the possible health states resulting from diagnosis and subsequent therapy. These can be determined by eliciting from the patient the numerical values of the possible dental health conditions using a visual analogue scale technique. Clinical decision analysis can then be carried out to calculate the most appropriate diagnostic strategy for the patient. Clinical decision analysis is starting to influence the development of guidelines for the diagnostic use of radiographs although its application in dentistry needs further refinement and development.