(1) Intrathoracic petechiae are characteristic of most SIDS cases, and tend to be more numerous in these cases than in deaths from other causes, including mechanical asphyxia. (2) Their localization suggests that intrathoracic negative pressure plays a role in their genesis. (3) Several human and animal studies suggest that petechiae arising from the pulmonary circulation may differ from those originating from systemic vessels in the thorax. (4) Experimental studies suggest that vigorous respiratory efforts are responsible for their formation. This would seem to exclude respiratory paralysis as a mechanism for most SIDS deaths. (5) These petechiae are not consistent with cardiac arrest or failure as the primary agonal event in SIDS. (6) The fact that petechiae are more prominent in SIDS than in other deaths adds to the evidence that SIDS is not a "wastebasket" diagnosis, despite the imperfection of existing diagnostic criteria. (7) These petechiae do not prove that the final mechanism of most SIDS deaths is upper airway obstruction. However, they do provide substantial support for that thesis.