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Sudden infant death syndrome (SIDS) still accounts for considerable numbers of unexpected infant deaths in many countries. While numerous theories have been advanced to explain these events, it is increasingly clear that this group of infant deaths results from the complex interaction of a variety of heritable and idiosyncratic endogenous factors interacting with exogenous factors. This has been elegantly summarised in the "three hit" or "triple risk" model. Contradictions and lack of consistencies in the literature have arisen from diverse autopsy approaches, variable applications of diagnostic criteria and inconsistent use of definitions. An approach to sudden infant death is outlined with discussion of appropriate tissue sampling, ancillary investigations and the use of controls in research projects. Standardisation of infant death investigations with the application of uniform definitions and protocols will ensure optimal investigation of individual cases and enable international comparisons of trends.

The present medico-legal investigation of the deaths of infants less than one year of age was designed to enhance our understanding of the Sudden Infant Death Syndrome (SIDS). Special emphasis was placed on the significance of cardiovascular pathology (including heart weight, malformations and myocarditis); exposure to nicotine; and mutations and/or polymorphisms in mitochondrial DNA (mtDNA). A total of 505 infants investigated at the Department of Forensic Medicine in Stockholm, between 1980 and 2001 were studied retrospectively. Body weight was found to be the best indicator of heart weight. A reference table for assessment of the heart weight as a function of the body weight is presented. Heart weights of SIDS victims did not differ from that of the other infants, with the exception of cardiovascular malformations. However, the data do strongly suggests that an increased heart weight, particularly greater than the 75th percentile, should be regarded as an exclusion criterion for SIDS. Congenital cardiovascular malformations (CCMs) were present in 18 infants (3.9 %) of the infants involved in this study. Surprisingly, 67 % of these CCMs, which were either the single, or the contributing cause of death were not diagnosed prior to death. The fact that undiagnosed and most likely operable CCMs as a cause of sudden unexpected death in apparently healthy infants indicates a need for more frequent examinations during the postnatal period. Myocarditis was detected in 16.8 % of natural deaths and 7.4 % of violent deaths. This condition was acute in 46, and chronic in 25 infants. Approximately 35 % of the infants with CMV exhibited myocarditis. With regards to fatality, the location of the inflammatory cells appeared to be more important than the number of foci. Foci of inflammatory cells in the interatrial septum and upper part of the interventricular septum appear to be most lethal, probably because parts of the conduction system are present in this area. High pericardia