To identify how British Child Death Overview Panels (CDOPs) and paediatric pathologists classify cause of death for sleep-related Sudden Unexpected Death in Infancy (SUDI). To determine compliance with national requirements for SUDI investigation. Electronic survey of CDOPs and pathologists using three vignettes of SUDI cases illustrating: accidental asphyxia, typical Sudden Infant Death Syndrome (SIDS) and SIDS with co-sleeping. Thirty-eight (41%) of 92 CDOPs returned questionnaires, and 32 were complete. Thirteen (14%) of 90 pathologists returned complete questionnaires. Thirty-one (97%) CDOPs and 7 (53%) pathologists agreed with the cause of death in the accidental asphyxia case; 24 (75%) CDOPs and 9 (69%) pathologists in the typical SIDS case; and 11 (34%) CDOPs and 1 (8%) pathologist in the co-sleeping SIDS case. Pathologists used the terms SUDI or unascertained as the cause of death for the accidental asphyxia case (46%) and the co-sleeping SIDS case (77%). These terms were used by CDOPs for the typical SIDS case (25%) and the co-sleeping SIDS case (41%). Seventeen (46%) CDOPs reported compliance with guidelines for investigation in more than 75% of cases. There is wide variation in classification of deaths, with only limited agreement between CDOPs and pathologists. The terms SIDS and accidental asphyxia are underused, even in typical cases.