Glucokinase plays an important role in glucose metabolism in pancreatic beta-cells and liver. Recently, several mutations responsible for noninsulin-dependent diabetes mellitus (NIDDM) have been identified within the coding regions of the glucokinase gene. We screened the promoter regions using polymerase chain reaction followed by single strand conformation polymorphisms in 240 Japanese NIDDM and 111 control subjects. In the beta-cell promoter, two kinds of sequence variations were detected. One variation, in which 2 nucleotides at position -282 (C-->T) plus -194 (A-->G) were changed simultaneously, was found in 23 NIDDM (9.6%) and 12 control (10.8%) subjects. The other variation [e.g. -30 (G-->A)] was identified in 87 NIDDM (36.3%) and 40 control (36.0%) subjects. In the liver promoter, in addition to the -603 (G-->T) substitution in 1 NIDDM (0.4%) and 2 control (1.8%) subjects, the -120 (G-->T) substitution in 1 control (0.9%) subject was found. However, there were no differences in these allele frequencies between NIDDM and control subjects. We conclude that the prevalence of mutations in the promoter of the glucokinase gene responsible for NIDDM is rare among Japanese patients.