Background: Studies on the ecology of medical care have been reported only from the USA. No similar investigation has been made for Japanese population. Objective: To sketch health care seeking behavior of people in Japan based on a prospective health diary recorded for one month. Material and methods: A population weighted random sample from a nationally representative panel of households was used to estimate the number of health-related symptoms, self-care, and health care utilization per 1,000 Japanese population per month. Variations in terms of age, sex, and region were also examined. Results: Based on 1,286 households (3,477 persons: 2,451 adults and 1,026 children), on average per 1,000 persons, 862 had at least one symptom, 307 visited a physician's office, 232 a primary care physician, 88 a hospital-based outpatient clinic, 49 a professional provider of complementary or alternative medical care, 10 a hospital emergency department, and 6 a university-based outpatient clinic. Seven were hospitalized, 3 received professional health care in their home, and less than 1 was admitted to a university hospital. Children had more physician and emergency visits, and rural people were more likely to be hospitalized compared with the average figures. Females were more likely than males to have symptoms and to visit their physician while the reverse was true for emergency visits. Conclusions: Compared with the data from the USA, more people visit physician offices and hospital based outpatient clinics in Japan. Results of this study would be useful for further delineation of health care seeking behavior of people in the context of a health care system unique to Japan.