Aim Along with rapid aging, medical expenditure for older adults has been increasing in Japan. Research has shown that periodontitis is a useful predictor for excess medical expenditure; however, limited information is available on the elderly population after adequately considering confounding factors. The aim of the present study was to evaluate the association between periodontitis and long‐term medical expenditure in elderly Japanese. Methods Baseline health and periodontal examinations were carried out in June 2008. Japanese adults (n = 245) aged 80 years were classified into quartiles based on periodontal inflamed surface area (PISA), which quantifies the degree of periodontal inflammation. Medical care use and costs were monitored by assessment of the National Health Insurance claim files from the baseline survey through the end of February 2011. Multivariable analysis of the differences in medical expenditure among PISA quartiles was carried out using linear regression with robust standard errors. Results The participants in the fourth (with the largest PISA) and third quartiles had significantly higher inpatient medical expenditure compared with those of the first quartile (P < 0.01 and = 0.04, respectively). Participants in the fourth quartile had significantly higher total medical expenditure (P < 0.01) compared with the first quartile. A trend was observed of higher inpatient and total medical expenditure with increasing PISA. Conclusions A significant association was found between periodontitis and future increase in medical expenditure, suggesting that periodontitis might be a modifiable factor for the reduction of excess medical expenditure among elderly Japanese. Geriatr Gerontol Int 2016; 16: 856–864.