Non–insulin–dependent (type 2) diabetes mellitus (NIDDM) is a common disorder of middle–aged individuals characterized by high blood glucose levels which, if untreated, can cause serious medical complications and lead to early death. Genetic factors play an important role in determining susceptibility to this disorder. However, the number of genes involved, their chromosomal location and the magnitude of their effect on NIDDM susceptibility are unknown. We have screened the human genome for susceptibility genes for NIDDM using non– and quasi-parametric linkage analysis methods in a group of Mexican American affected sib pairs. One marker, D2S725, showed significant evidence of linkage to NIDDM and appears to be a major factor affecting the development of diabetes mellitus in Mexican Americans. We propose that this locus be designated NIDDM1.