Mitochondrial tRNA(Leu)(UUR) gene mutation is one of the candidates in the pathogenesis of NIDDM. Especially the 3243 (A-->G) mutation is associated with the maternally-inherited diabetes and deafness. To evaluate the prevalence and characteristics of the 3243 point mutation in Koreans, we screened 433 Korean diabetic patients (220 men and 213 women). Genomic DNA was extracted from peripheral white blood cells and PCR was carried out with mitochondrial DNA primers (3130-3149, 3558-3539) encompassing the 3243 position. After digestion with Apa-1, five subjects showed polymorphism suggesting 3243 point mutation but when we directly sequenced the amplified DNA with an automatic sequencer, only 2 of the 5 patients were shown to have 3243 (A-->G) mutation and the other 3 subjects had 3426 (A-->G) mutation rather than 3243 mutation. Two diabetic patients with 3243 mutation were lean (BMI = 14.4, 17.0 kg/m2), had relatively lower fasting C-peptide concentrations (0.9 ng/ml each), and required insulin for management. In contrast, those with 3426 point mutation were not lean (BMI = 22.6-28.0 kg/m2), had relatively higher C-peptide levels (3.9-5.4 ng/ml), and could be managed with oral hypoglycemic agents. None of the 5 patients had deafness. In conclusion, the prevalence of 3243 point mutation in Korean diabetic patients was approximately 0.5% and we found a new mutation mimicking 3243 mutation by PCR-RFLP (restriction fragment length polymorphism) pattern. We suggest that sequencing of the PCR product or designing smaller PCR fragment size to enhance the specificity may help to identify the exact location of the point mutation.