A pedigree with maternally transmitted diabetes mellitus, deafness, and cardiomyopathy is described. A A-->G mutation at nucleotide pair 3243 in mitochondrial gene was detected by Apa I digestion of PCR amplified genomic DNA from 3 brothers and their mother. The proband, suffering from CHF, showed unique fine granular pattern of hyperechogenic cardiomyopathy as his brother and their mother did. Although he is recently treated with insulin, he was initially NIDDM treated by sulfonylurea. His urinary CPR excretion decreased gradually to as low as less than 10 micrograms/day in these 3 years. The insulin response to oral glucose was decreased in all other family members with the mutation. It is suggested that the defective insulin secretion exists in this family with the mutation and the progressive decrease in insulin secretion might resulted in IDDM in the proband.