Purpose : This study was to identify the incidence, trend, and situation of the unexpected sudden death of elementary, middle, and high school students in school for 17 years from 1988 to 2004 in seoul. Methods : We used the data of The Seoul Safety Mutual aid Association in seoul. Subjects were 53 unexpected sudden death in school from elementary, middle, and high school students, and we explored the gender, month, a day of the week, year, contents of situation, and medical certificate of death. Statistic analysis were chi-square test, Cochran-Armitage trend test, and the tool was SAS 9.1.Results : There were 53 unexpected sudden death students(male 42 and female 11). The incidence of male students was higher than that of female during 16 years except in 1995. The incidence occurred in middle school students were 31 (58.5%) of 53 and were also most frequent in middle school students in both of male and female students. In 1990, the incidence was 8(15.1%) students, 6(11.3%) in 1992 , and 5(9,4%) in 2000 respectively. The frequency of unsuspected sudden death in March was 11(20.8%) of 53 students, 11 (20.8%) in September, 7(13.2%) in October. In Thursday, it was 12(22.6%) of 53 students. Only 14(26.4%) students of 53 died during general life, but 39(73.6%) were related to excercise. The diagnosis of 14 students died during general life were cardiac arrest 7(50.5%), brain disorder 3(21.4%) which were based on medical certificate. But the diagnosis of 39 students died during or after excercise were brain disorder including cerebral hemorrhage 9(23.1%), heart disease 9(23.1%), cardiac arrest 8(20.5%), and unknown 6(15.4%), respectively. Conclusion : The incidence of unexpected sudden death were more frequent in male students, in middle school, and in excercise-related situation and the trend was similar for 17 years. Therefore, to prevent the unexpected sudden death, it needs to further study substantially the risk factors of unexpected sudden death including past history, life-style, nutrition and development, family history, and learning environment.