Since plaque plays a major rolein the development of periodontal inflammation, special attention is paid to its composition and quantity. Several indices have been designed for the quantification of the supragingival plaque. The aim of the present study was to compare the utility and discriminating power of five such plaque indices. In 14 subjects, three teeth were selected randomly on which the undisturbed plaque formation was studied over a period of 96 hours. This examination was performed twice, the first time after induction of gingivitis and a second time starting with a healthy gingiva. It is known that in the presence of gingival inflammation, the plaque growth rate slightly increases. Plaque formation was estimated at several visits with the following indices: Harrap index, Quigley and Hein index, Navy index modified by Clemmer and Barbano, Navy index modified by Hancock and Wirthlin, and a planimetrical plaque scoring system. The path of the plaque growth curve was found to depend on the selected plaque index, going from an exponential curve (planimetry) over an sigmoidal curve (Harrap index, Navy index modified by Hancock and Wirthlin) to a nearly linear relation (Quigley and Hein, Navy index modified by Clemmer and Barbano). By comparing the plaque scores on patients with healthy gingiva with those from patients with inflamed gingiva, it was found that the planimetrical plaque index had a clearly higher discriminating power. It was concluded that the choice of an index for the measurement of the plaque extension depends on the goals of the study.J Periodontol 1991; 62:100–105.