Aim: To examine the diversity of Escherichia coli serotypes found in the intestinal contents of infants who died of Sudden Infant Death Syndrome (SIDS) compared with that in comparison infants. Methods and Results: Over the 3-year period, 1989-1991, in South Australia and Victoria (Australia), a total of 687 E. coli isolates from 231 patients with SIDS (348 isolates), 98 infants who had died from other causes (144 isolates) and 160 healthy infants (195 isolates) were studied. The isolates from patients with SIDS were found to represent 119 different serotypes; the isolates from 'other cause' infants represent 97 different serotypes; and the isolates from healthy infants represent 117 different serotypes. The seven common serotypes isolated most frequently from infants with SIDS belonged to those associated with extra-intestinal infections in humans. Compared to healthy infants (6%), these were found in significantly higher proportions among infants who died of other causes (13%, P < 0 center dot 05) or infants with SIDS (18 center dot 7%, P = 0 center dot 0002). Conclusions: Despite these sources yielding a wide variety of serotypes of E. coli, a pattern of certain potential pathotypes of E. coli being associated with SIDS is apparent. Significance and Impact of the Study: While SIDS remains one of the most important diagnoses of postneonatal death, its causes are still unexplained. If E. coli has a role in the pathogenesis of SIDS (as suggested by the pathotypes identified on the basis of serotype), further studies may reveal novel virulence factors that may clarify the role of this bacterium in SIDS.