This paper presents the clinical and metabolic findings in two young boys with long-standing Kearns-Sayre syndrome. Following short exposure to oral prednisone, both boys developed lethargy, increasing somnolence, polydipsia, polyphagia, and polyuria. Both presented in the emergency room with profound coma, hypotension, severe hyperglycemia, and acidosis. Nonketotic lactic acidosis was present in one and ketosis without a known serum lactate level was present in the other. Respiratory failure rapidly ensued and both patients expired in spite of efforts at resuscitation. We believe these two cases represent a newly described and catastrophic metabolic-endocrine failure in the Kearns-Sayre syndrome.