Sleep apnea syndrome is defined as the repetitive, prolonged cessation of air flow associated with hypoxemia, hypercapnia, and acidosis during sleep. A widely accepted criterion for SAS is more than 30 episodes of apnea (lasting 10 seconds or more) during 7 hours of sleep. In the case examined, a 24-year-old obese woman was found dead in her boyfriend's apartment in his absence. She had been admitted to the hospital six times previously because of diminished consciousness, respiratory failure, and pneumonia. A diagnosis of obesity-sleep apnea (Pickwickian) syndrome was made. An autopsy showed that she had an extremely small larynx, intra-alveolar hemorrhage, edema, pulmonary lymphocyte infiltration, and severe focal myocardial fibrosis. No fresh myocardial lesion, coronary arterial lesion, or findings of heart failure were determined. The woman's elder sister had also died of the same disease at the age of 23. The cause of death was diagnosed as respiratory failure and pneumonia with the SAS as the underlying cause of death. Although no autopsy reports of the SAS have been published in the field of forensic pathology, this syndrome is a predominant cause of sudden death in obese persons and could be a hidden cause of accidental death in such persons. 4 figures and 16 references