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We sought to describe the characteristics and sleep circumstances of infants who die suddenly and unexpectedly and to examine similarities and differences in risk factors among infants whose deaths are classified as resulting from sudden infant death syndrome (SIDS), suffocation, or undetermined causes.We used 2005 to 2008 data from 9 US states to assess 3136 sleep-related sudden unexpected infant deaths (SUIDs).Only 25% of infants were sleeping in a crib or on their back when found; 70% were on a surface not intended for infant sleep (e.g., adult bed). Importantly, 64% of infants were sharing a sleep surface, and almost half of these infants were sleeping with an adult. Infants whose deaths were classified as suffocation or undetermined cause were significantly more likely than were infants whose deaths were classified as SIDS to be found on a surface not intended for infant sleep and to be sharing that sleep surface.We identified modifiable sleep environment risk factors in a large proportion of the SUIDs assessed in this study. Our results make an important contribution to the mounting evidence that sleep environment hazards contribute to SUIDs.