Objective To develop a detailed understanding of the circumstances of sudden unexpected death in infancy (SUDI) cases subject to serious case review (SCR).

Design This was a thematic analysis of SCRs relating to cases of SUDI in England. SCRs were obtained for SUDI cases dying between 1 April 2011 and 31 March 2014. These were cases (aged 0–2 years) that presented as a SUDI and for which no clear medical or forensic cause of death was found.

Results SCRs were held for 30 SUDI cases, published reports were available for 27/30. The median (range) age at death was 2 (0–19) months. Background risk factors in families included: alcohol or drug dependency in 18/27, parental mental health problems in 14/27, domestic abuse in 9/27 and parental criminal records in 13/27. Nineteen infants had received support from social care, 10/19 were subject to child protection plans. Neglect was a feature in 15/27 cases. Parents did not engage with professionals in 18/27 cases, involving social care in 14/18, health care in 13/18 and drug and substance misuse services in 5/18. Eighteen of 27 deaths occurred in highly hazardous sleep environments, 16/18 involved cosleeping and 13/16 cosleeping deaths occurred with parents who were intoxicated with alcohol or impaired by drugs.

Conclusion Most SUDI cases occurred in hazardous sleep environments and are potentially preventable. They occurred in families well known to services with concerns about neglect, substance misuse and poor engagement. More consideration is needed on how best to support such vulnerable families.