Thirty patients with adult periodontitis were monitored using an automated periodontal probe for 6 months in order to determine the prevalence of active sites and the overall pattern of disease progression in active sites. The automated probe is capable of measuring probing attachment levels relative to the cemento-enamel junction with better than 0.2 mm of accuracy. The prevalence of disease activity was dependent on the threshold for probing attachment loss used. When the smallest threshold (0.4 mm) was used the prevalence of active disease was 29%, whereas a large threshold (2.4 mm) detected only 2% active sites. Regression analysis of the active sites revealed that 76% of the sites lost probing attachment consistent with a continuous model for disease progression. A small subset of sites demonstrated either bursts of activity or exacerbations and remissions of disease activity.