Background. 19.6 % of all fatal cases among Russian children who died in the first year of life are registered out-of hospital, and they are usually sudden. Out-of-hospital sudden unexpected infant death (OSUID) could be confused with sudden infant death syndrome (SIDS) followed by the wide variation in the diagnostic indicators of the syndrome.The aim of the study is to reveal the pathological changes in the tissues and organs of 420 cases of OSUID (group 1) compared with 350 cases of death in the hospital (group 2), to estimate the conformity of the final diagnoses and to identify cases of OSUID, which were SIDS.Methods. From 2009 to 2016 pathoanatomical and virological study of the organs of 420 cases of OSUID (group 1) and 350 babies wh...