Restriction fragment length polymorphism (RFLP) is a usual and fundamental method for the detection of disease and identification of its pathogenesis on the molecular level. The genetic background of noninsulin dependent diabetes mellitus (NIDDM) has been strongly suggested, but its precise nature was obvious because many other expression factors concern its expression, We first investigated insulin gene RFLP in 43 NIDDM patients and 11 non-diabetics in Korea. We found 91 and 86 percent of the genotype 1/1 in NIDDM and nondiabetics respectively. Also genotype 1/3 was found 9 and 14 percent each in both groups. Genotype 3/3 was not detected in two groups. Four acromegalics who had secondary DM showed only genotype 1/1. These findings showed that class 3 allele of insulin gene was a weak genetic marker of NIDDM, and the method for detection of RFLP is the fundamental tool in clinical genetic research.