OBJECTIVE We examined the impact of household income on the use of medical services in Japan, where there is a "health care for all" policy, with important, centralized influence by the national government designed to ensure universal access. METHODS All healthcare societies operating in 2003 were included in the study, representing 14,776,193 insured adults and 15,496,752 insured dependents. The mean case rate (the average number of monthly bills per patient), the mean number of service days per person, and the mean medical cost per person served as indicators of medical service use. Multiple regression analysis was performed by the forced entry method using case rate, the number of service days, and medical cost as outcome variables, and average monthly salary, dependent ratio, average age, and premium rate as the explanatory variables. RESULTS In the multiple regression analyses, average monthly salary showed a high positive correlation of outpatient and dental indicators, including case rate, the number of service days, and medical cost. If the average monthly salary were reduced 20 percent lower than the mean, the estimated changes (95 percent CI) in case rate for the insured were -7.49 (-8.14 approximately -6.84) percent for outpatient visits and -8.16 (-8.77 approximately -7.56) percent for dental services. CONCLUSIONS Average monthly salary intensifies the effects of copayments on the case rate, the number of service days, and medical cost in the "Employees Health Insurance" in Japan. Thus, a low salary appears to discourage patients from seeking medical and dental services.