Objectives This study demonstrated the relationship between experience as a health promotion volunteer (Hoken-hodouin) and medical costs in Japan. The study area was Suzaka City (March 2016 population: 51,637) in Nagano Prefecture, Japan, where a total of about 300 women have been engaged and trained as health promotion volunteers since 1958.Methods A cross-sectional survey was conducted in 2014 using a self-administered questionnaire, which included items on experiences as a health promotion volunteer, age at engagement, leadership status, and satisfaction with the experience. Eligible study participants were all residents of Suzaka aged 65 years or over. Medical cost data from April 2013 to March 2014 were collected for women aged 65-74 years who were beneficiaries of the Japanese National Health Insurance (n=2,304). Medical consultation rates and costs for treatment at outpatient and inpatient clinics were analyzed as outcomes. Adjustments were made for age, marital status, educational level, cohabitation status, equivalent income, alcohol use, smoking status, awareness about a healthy diet, and walking time per day.Results Of the 2,304 study participants, 1,274 (55.3%) had experience as health promotion volunteers. Poisson regression analysis revealed that volunteers' experience was positively associated with outpatient care rates (adjusted relative risk [RR]=1.04; 95% confidence interval [CI]=1.02-1.07), and negatively associated with inpatient care rates (RR=0.74; 95% CI=0.56-0.98). Multivariate regression analysis revealed that the adjusted geometric means of outpatient and inpatient care costs were 7% and 23% lower, respectively, among participants with volunteer experience than that among those with no volunteer experience (140,588-151,465 JPY for outpatient costs; 418,457-539,971 JPY for inpatient costs). These associations were stronger among participants who began health promotion volunteer at age 60 years or more, those who had leadership roles, and participants who had high levels of satisfaction regarding their volunteer experience.Conclusion These results suggest that experience as a health promotion volunteer is associated with lower medical costs, particularly for inpatient care.