Maternally inherited diabetes and deafness (MIDD) and
mitochondrial encephalomyopathy with lactic acidosis and
stroke-like episodes (MELAS) are different syndromes,
but are caused by the same m.3243A>G mutation in
mitochondrial DNA. Why some patients develop MIDD
while others MELAS is unknown, but may be related to
heteroplasmy level. Progression from MIDD to MELAS has not been described. Here we report a patient with MIDD who over time developed severe insulin resistance and symptoms and signs consistent with MELAS. The most likely explanation here was paternal co-inheritance of type 2 diabetes in combination with a high heteroplasmy level. The present case showing evolution of MIDD to MELAS supports the concept that both syndromes can be regarded as two phenotypes of the same disease.