The A-G point mutation of mitochondrial tRNALeu(UUR) gene at nucleotide 3243 caused clinical NIDDM in combination with or without deafness and was inherited through maternal transmission. An ApaI restriction site was resulted by this mutation, which could be detected by polymerase chain reaction (PCR) with ApaI digestion and be used in clinical genetic diagnosis. The first Chinese family with this mutation found through our screening of NIDDM patients was reported. Genetic diagnosis was done in 12 of 15 members in this three-generation pedigree. Positive results (present of ApaI restriction site) were observed in all 5 NIDDM patients (four with deafness). In 6 of 7 non-diabetic members, negative genetic diagnosis was obtained. The presymptomatic diagnosis of this disease was made in a 13-year old non-diabetic boy with positive genetic diagnosis in conjunction with the maternal segregation pattern of the mutation in this family. The finding of this disease in Chinese provides firm evidence of genetic heterogeneity in Chinese NIDDM. The techniques of the genetic diagnosis of this disease can be used in clinical laboratory, which indicates that in the diagnosis of NIDDN, the molecular etiologic level can be reached in daily clinical practice.