Island regions suffer from a shortage of health care in part because they are less developed, they cover a widespread area relative to the population, and due to transportation barriers. The purpose of this study was to assess the level of illness and the magnitude of medical care utilization, and to investigate the determinants of utilization in these area. The data were collected by means of a household survey conducted from February 16 to 25, 1987 on S islands which were selected in consideration of the size of the population, the distance from the main land, and the distribution of health care facilities. The household response rate was 89.1% (491 of 551 households), and 1971 persons were surveyed. The major findings of this study are as follows: 1) The morbidity rate of the island inhabitants was 27.7% during the two weeks, and 25.5 chronic illnesses and 9.1 acute illnesses per 100 persons, were noted. Differences in the magnitude of illness were statistically significant by sex, age, education, and family size. 2) The magnitude of total ambulatory carl utilization was 16.8 visits per 100 persons during the two weeks, which was less than that of other regions; and differences in the magnitude of total ambulatory care were statistically significant by sex, age, education, occupation, and family size. 3) Unmet needs were classified as 56.0% in chronic illnesses and 19.6% in acute illnesses; and differences in unmet needs were statistically significant by sex, age, education, occupation, income, and family size. 4) Statistically significant determinants in medical care utilization included the frequency of acute illness and chronic illness, and income in total utilization; the frequency of chronic illness and acute illness, and medical care insurance in physician visits. 5) According to the results of the path analysis, need factors had the greatest effect on utilization, and predisposing factors had more indirect effects through enabling or need factors than direct effects.