A computerized planimetric method for plaque area measurement has been developed. Using a computerized image analysis system (CIAS), the plaque area and tooth area on color slides were digitized and the number of pixels automatically counted. The proposed Plaque Percent Index (P% Index) expresses plaque area as a percentage of tooth area. The reproducibility of this method was tested and the influence of photographic technique on the P% Index was determined. The association of the Turesky modified Quigley-Hein plaque index (Q-H Index) and the P% Index was assessed. The present method was highly reproducible for the P% Index with an intraexaminer variation of 0.28% and intraexaminer correlation coefficient of 0.99. The results show that highly reproducible P% Index values with an error of less than 3.0% were obtained when the photographs were taken under the following conditions. For the buccal surfaces of anterior teeth, the slides were photographed within a 20-degree range in the horizontal plane and a 30-degree range in the vertical plane; and for the lingual/palatal surfaces of anterior teeth and the buccal or lingual/palatal surfaces of posterior teeth, the slides were photographed with an image of the whole tooth surfaces in the photographic mirrors. The comparison of the Q-H Index and the P% Index revealed that for each score of the Q-H Index the corresponding values of P% Index were wide with a significant crossover value, although a strongly positive correlation was found between the Q-H Index and the P% Index (r = 0.92, P < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)