Abstract Background. Incidence of sudden unexpected death in infancy (SUDI) and sudden infant death syndrome (SIDS) differs among studies and non-autopsied cases are difficult to assess. Objectives. To investigate causes of sudden death in infancy in a nationwide setting. Validate the use of the ICD-10 code for SIDS (R95) in the Danish Cause of Death registry. Design. A retrospective analysis of all infant deaths (<1 year of age) in Denmark in 2000-2006. All death certificates and autopsy reports were read. Results. We identified 192 SUDI cases (10% of total deaths, 0.42 per 1000 births) with autopsy performed in 87% of cases. In total, 49% of autopsied SUDI cases were defined as SIDS (5% of all deaths, 0.22 per 1000 births); Cardiac cause of death was denoted in 24% of cases. The Danish Cause of Death Registry misclassified 30% of SIDS cases. Conclusions. A large proportion of infant deaths are SUDI, and the majority of these are caused by cardiac disease or SIDS. Autopsy is not always performed and valuable information is subsequently lost. Cause of Death registry data is not accurate in describing SIDS.