The microscopic distribution of thoracic visceral petechiae in 100 consecutive victims of sudden infant death syndrome (SIDS) was retrospectively analyzed. Thymic and cardiac petechiae were more numerous near their surfaces than in the central parenchyma, suggesting that abnormal intrathoracic pressure changes were important in their pathogenesis. Low intrathoracic pressures may have preferentially distended, to the point of rupture, the outer rather than central microvasculature of these two organs. The distribution of pulmonary petechiae in association with lung congestion and edema indicated left ventricular failure. Low intrathoracic pressures generated by breathing against an occluded airway could have caused heart failure by increasing the afterload and decreasing the compliance of the left ventricle. These petechiae occur in more than 80% of SIDS cases, suggesting that abnormal intrathoracic pressure, perhaps caused by breathing against an obstructed airway, is a common terminal event.