The accurate and reproducible measurement of gingival attachment level is essential for studying the progression of periodontitis in human populations. An automated periodontal probe has been developed which measures gingival attachment level using the occlusal or incisal surface of the tooth as a fixed landmark and which transfers data directly to a computer. The probing force is adjustable and is regulated by changing the air pressure. A mercury level switch maintains uniform probe angulation during measurements. The reproducibility of measurements obtained with the automated periodontal probe was examined by making duplicate gingival attachment level measurements at 784 sites in 9 patients 1 week apart and comparing the difference between the paired values. A probing force of 0.30 N was used. There was no significant difference in the frequency distributions of measurement differences between either maxillary versus mandibular teeth or anterior versus posterior teeth. The data demonstrate that this device can be used to measure gingival attachment level without the need for stents or estimation of the level of the cementoenamel junction and that reproducible data can be obtained and automatically stored in a computer for subsequent analysis.