Many studies have examined the presence of Porphyromonas gingivalis in periodontal pockets. However, monitoring the number of bacterial cells is difficult. In this study, we performed quantitative analyses of P. gingivalis to clarify the relationship between the numbers of this organism and periodontal status. Using the TaqMan real-time PCR system, we found a significant positive correlation (P < 0.0001) between the number of P. gingivalis and pocket depth. The slope of the regression line indicated that for every 1-mm increase in pocket depth, the number of P. gingivalis increased 10- fold. There was also a significant reduction (P < 0.01) in the numbers of P. gingivalis before and after treatment. These results suggest that the absolute and relative numbers of P. gingivalis are closely associated with periodontal status, and that quantitative analysis of this organism is important for the evaluation of periodontal therapy.