We present 2 cases of progressively severe Kearns-Sayre syndrome (KSS) with multisystemic affectation and atypical endocrine and cutaneous features, a 16-year-old patient (case 1) and a 5-year-old patient (case 2). Endocrine studies showed high glucose and glycohemoglobin concentrations with normal pancreatic reserve and low values of ACTH, cortisol, LH and FSH in case 1. Normal ACTH values with low concentrations of cortisol and PTH were observed in case 2. Southern blot analysis and PCR amplification revealed the presence of a deletion of approximately 6.7 kb in the mitochondrial DNA of both patients. Endocrinological studies suggest that adrenal insufficiency may be an additional feature of KSS that worsens the clinical evolution of the patients. In spite of a normal pancreatic reserve, insulin therapy should be considered in patients with diabetes mellitus of mitochondrial origin.