The prevalence of diabetes mellitus in Korea has increased tremendously during the past several decades. In addition to the explosive increase of diabetes mellitus, the clinical characteristics of diabetes mellitus in Korea seems to be quite different from those of Western countries. Most patients with non-insulin-dependent diabetes mellitus (NIDDM) are non-obese and many of them lose weight significantly during the course of developing diabetes. The pathogenic mechanism(s) underlying this phenotype is still unclear. A subgroup of patients known as slow onset IDDM exists, but this could not explain the majority of non-obese NIDDM in Korea. In order to examine whether insulin resistance/hyperinsulinaemia is important in the development of NIDDM we conducted the following studies. Fasting plasma glucose and insulin concentration were measured in subjects with normal glucose tolerance, impaired glucose tolerance and diabetes mellitus. The peak plasma insulin level was much lower than that of Pima Indians or Caucasians. We also found that the insulin sensitivity index measured by euglycemic hyperinsulinaemic clamp in the first degree relatives of Korean NIDDM patients was not lower than that in a control group. This data suggests that insulin resistance, or hyperinsulinaemia, does not play a major role in the development of NIDDM in Korea. Taken together with the rarity of obese NIDDM in Korea, it maybe suggested that the insulin secretory capacity of Korean people being smaller than Western people they cannot compensate for the insulin resistance imposed by recent changes in life style.