The objective of this study was to provide a predictive tool to assist forensic and pediatric pathologists in the diagnosis of sudden unexpected infant death and to discuss the pathogenesis of intrathoracic petechial hemorrhages through a retrospective autopsy report review of 174 sudden infant death syndrome (SIDS) cases (2004 definition) and 67 age-matched comparison deaths. The setting was a qualitative assessment (presence or absence) of macroscopic intrathoracic petechiae in SIDS and age-matched comparison of sudden unexpected deaths that occurred in the late 1980s and early 1990s. Sensitivity, specificity, and positive and negative predictive values for thymic, pleural, and epicardial petechial hemorrhages were developed. Results showed 89.5%, 80%, and 79.9% SIDS (<12 months of age) had thymic, pleural, and epicardial petechiae, respectively, compared with 47.6%, 47.5% and 43.6% in non-SIDS deaths, respectively. Respective odds ratios were: 9.4 (4.5 to 19.9), 4.6 (2.3 to 9.1), 5.3 (2.6 to 10.8). When all 3 intrathoracic organ sites contain macroscopic petechiae, this is 84.9% predictive of SIDS; when all 3 sites have no detectable petechiae this is 93.1% predictive of a non-SIDS diagnosis. Thus, we conclude that careful assessment of intrathoracic petechiae at autopsy is likely to be diagnostically useful in the assessment of sudden unexplained infant death.