Kearns-Sayre syndrome (KSS) is a rare mitochondrial disease in which neuromuscular structures, endocrine glands, and cardiac conduction systems are most commonly involved. An 11-year-old boy was admitted with blurred consciousness, respiratory instability, and bradycardia of two-hour onset. He was immediately intubated. His medical history included growth retardation and myopic refractive defect for six years, therapy for somatomedin C deficiency for 15 months, and bilateral ptosis for three months. On physical examination, he was unconscious, had a peak heart rate of 40/min, blood pressure of 60/20 mmHg, and a weak pulse. Laboratory findings showed elevated blood lactate and blood pyruvate levels and an increased lactate/pyruvate ratio. The electrocardiogram showed complete atrioventricular block and echocardiography showed mitral valve prolapse. Following implantation of a temporary transvenous cardiac pacemaker, his heart rate and clinical condition improved. Further analysis with cranial magnetic resonance (MR) imaging demonstrated hyperintense signal changes in the subcortical white matter of the two cerebral hemispheres, bilateral thalamus, putamen, cerebral peduncles, dorsal medulla, and midbrain. The typical clinical and MR findings confirmed the initial diagnosis of KSS. A permanent cardiac pacemaker was implanted into the right ventricle.