Background We aimed to describe the ecology of medical care on an isolated island with limited access to secondary care, and to evaluate the gatekeeping function of the island’s primary care clinic through comparison with a previous nationwide survey.

Methods We conducted this retrospective, open cohort study on Iheya, an isolated island in Okinawa Prefecture that has one primary care clinic. We considered Iheya as unique location in which to examine the role of primary care in Japan. Participants were patients who visited the island’s clinic between February 1, 2013 and January 31, 2014. We calculated the number of visits to the clinic and referrals to off-island medical facilities using electronic medical records. We also compared data for Iheya with a nationwide survey conducted in 2003.

Results Iheya had 1314 inhabitants in 2013. Of the 5682 visits to the clinic in the 1-year study period, 290 people were referred to off-island medical institutions. There were 64 referrals to emergency departments; of these, 57 people were admitted to hospital. The rate of visits to the clinic per month per 1000 inhabitants was 360.4 visits (95% confidence interval: 351.0–369.7). Of these, 18.4 (16.3–20.5) were referred off-island, with 4.1 (3.1–5.1) referrals to emergency departments and 3.6 (2.6–4.6) hospitalizations. Despite the high incidence of visits to the primary care clinic, the rates of hospital-based outpatient clinic visits, emergency department visits, and hospitalizations were lower than rates reported in a previous Japanese study.

Conclusions This suggests that several dimensions of primary care, its gatekeeping function in particular, are likely to play important roles in this geographical setting.