This article investigates whether the decline in the sudden infant death rates and the rise in unascertained death rates during the period 1995-2003 were linked. It concludes that changes in certification practices surrounding sudden infant deaths and unascertained deaths suggest that it is becoming more difficult to distinguish between these two causes of death. In addition, there is a huge overlap in the characteristics of babies whose deaths are certified as sudden infant death and those whose death is unascertained. These terms are to some extent used interchangeably. Based on this evidence it seems appropriate to include both groups in any analysis of unexplained infant deaths.