Observer reliability in performing linear measurements between the cementoenamel junction and alveolar crest was determined for mandibular posterior teeth from digitized clinical bitewing radiographs acquired during recall examinations. 6 measurements (corresponding to traditional probing measurements) were made per tooth by 3 observers. Mesial and distal measurements made to the most coronal aspects of the alveolar crest were the most reliable and least biased. As was anticipated, intra-observer reliability was better than inter-observer reliability although the 3 observers of our study were able to detect a significant mean change (0.1 mm, p<0.0001) in alveolar bone height over a 1-year period for 10 patients. For our most reliable and unbiased measurements (mesial measurements to the alveolar crest), a change of 0.54 mm (90th percentile) would be required to indicate change at a site from one time to the next. Based on the reliability of our digital radiographic measurements, with the alpha error rate set at 0.05 and beta at 0.20, a difference in alveolar bone height of 0.3 mm could be detected with a patient sample size of between 13 (best case) and 54 (worst case).