Background Early detection of decreased kidney function can help prevent the progression of kidney disease to kidney failure and cardiovascular events. Potentially significant associations between kidney function and periodontal disease have been reported in cross-sectional studies. However, no longitudinal study has been performed and no study has been performed in Japan. The aim of this longitudinal study was to investigate the effect of periodontal disease on kidney function in community-dwelling Japanese elderly. Study Design Retrospective cohort. Setting & Participants Members of this cohort were drawn from a longitudinal interdisciplinary study of aging. Included for this analysis were 317 participants (166 men, 151 women) aged 75 years in 2003. Predictor The periodontal inflamed surface area (PISA), reflecting the amount of inflamed periodontal tissue, was calculated for each participant. Participants were classified in quartile groups according to PISA, then divided into 2 groups (highest quartile vs the other 3 groups combined). Outcomes The primary outcome for the analysis was decreased kidney function, defined as a decrease in estimated glomerular filtration rate at follow-up. Measurements Multivariable logistic regression analyses were performed to predict decreased kidney function on the basis of periodontal status, risk factors for kidney disease, and other potentially relevant covariates. Results During the 2-year follow-up (2003-2005), 45 participants (14.2%) developed decreased kidney function. The highest PISA quartile was associated significantly with a greater cumulative incidence of decreased kidney function (OR, 2.24; 95% CI, 1.05-4.79) than the referent group (the other 3 quartiles) after adjusting for covariates. Limitations Extension of interpreting the findings to other age groups is limited. Conclusion These results suggest that periodontal disease may be a risk factor for decreased kidney function in Japanese elderly.