MELAS syndrome (mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes) and MIDD syndrome (maternally inherited diabetes and deafness) are mitochondrial diseases caused in most cases by the same mutation m.3243A> G, which affects the gene MT-TL1. The cases of two families with MELAS are presented here. In the first case, the m.3243A>G mutation was detected and the heteroplasmy level in blood, urine and oral mucosa were determined, finding a great phenotypic variability: the patient had higher heteroplasmy in the three tissues compared against oligosymptomatic relatives, and the mother had high blood sugar levels and hearing loss, suggesting a phenotype near to MIDD. In the second family, the m.3271T>C mutation was detected, which constitutes the first case reported in Colombia. These findings suggest that MIDD and MELAS, often described as distinct entities, are part of the same entity with variable expressivity partially depending on heteroplasmy. -- El sindrome MELAS (encefalomiopatia mitocondrial, acidosis lactica y episodios similares a isquemia cerebral) y el sindrome MIDD (diabetes y sordera de herencia materna) son enfermedades mitocondriales producidas en la mayor parte de los casos por una misma mutacion: la m.3243A>G que afecta al gen MT-TL1. Se presentan los casos de dos familias con MELAS. En la primera se detecta la mutacion m.3243A>G y se determina el nivel de heteroplasmia en sangre, orina y mucosa oral, con lo que se evidencia una gran variabilidad fenotipica: la paciente tenia una mayor heteroplasmia en los tres tejidos en comparacion con sus familiares oligosintomaticos y la madre tenia una glicemia elevada e hipoacusia, sugiriendo un fenotipo cercano al MIDD. En la segunda familia se detecta la mutacion m.3271T>C, siendo el primer caso reportado en Colombia. Estos hallazgos sugieren que el MIDD y el MELAS, descritos frecuentemente como entidades distintas, hacen parte de una misma entidad con expresividad variable dependiendo en parte de la heteroplasmia.