The definition of sudden infant death syndrome(SIDS) is the sudden death of an infant that is unexpected by history and cannot be explained by a full postmortem examination including a review of medical history, investigation of the scene of death, and a complete autopsy. Recently, the possibility of occurrence of SIDS after immunization has been suggested. These speculations raised a public awareness as well as apprehension about immunization. SIDS is rare before 1 month of age, with a peak incidence at 2~4 months of age, and 95% of all cases occur by 6 months of age. There are many epidemiological factors associated with an increase of risk for SIDS, such as prone sleep position, maternal smoking during pregnancy, and inborn errors of metabolism However, immunization, which has recently become an issue in SIDS, has no relation to SIDS. The DTaP vaccine in particular, is given during the peak occurrence age of SIDS, which probably led to such misunderstandings. According to an epidemiological data on SIDS in Korea, the estimated incidence of SIDS in the year 1996 was 0.31 in 1,000 live births (male, 0.33 and female, 0.29) with 216 cases per year. The lower incidence of SIDS in Korea than in the United States, European countries, or Japan is probably due to the lack of a surveillance system and public cognition for SIDS. In the present, without established causes for SIDS, public education about the prevention of SIDS is more important than anything. Also, an efficacious surveillance system for SIDS is essential to prevent and decrease its incidence.