Background: Ectoenzyme nucleotide pyrophosphate phosphodiesterase 1 (ENPP1) is known to influence insulin sensitivity by inhibiting insulin receptor signaling. A DNA polymorphism in the ENPP1 gene at exon 4 (K121Q) was demonstrated to be associated with insulin resistance, type 2 diabetes mellitus (T2DM), and a risk of early myocardial infarction, albeit with controversy. Our aim was to investigate any association of ENPP1 K121Q alleles with T2DM, features of the metabolic syndrome, and diabetic cardiovascular complications in a Chinese population of Han origin. Methods: The ENPP1 K121Q polymorphism was determined by a restriction fragment-length polymorphismpolymerase chain reaction in 1,862 patients with T2DM and 844 non-diabetic subjects. Results: The genotype distributions or Q-allele frequency were not statistically different between the diabetic and non-diabetic groups. The anthropometric parameters, systolic and diastolic blood pressures, lipid profiles, and serum creatinine levels of subjects with different ENPP1 K121Q polymorphisms were not statistically different in the two groups or even in the pooled data. When sub-group analyses of diabetic subjects were stratified according to BMI levels (greater or less than 27), gender, age of diabetes onset (older or younger than 60 years), and the presence or absence of a diabetic family history; this polymorphism was still not associated with T2DM. Nor was the ENPP1 K121Q polymorphism associated with the prevalence of coronary artery disease and ischemic cerebrovascular disease in patients with T2DM. Conclusion: The ENPP1 K121Q polymorphism is not related to T2DM, features of the metabolic syndrome, or diabetic macrovascular complications in a Chinese population.