The clinical analysis was performed on 730 critically ill patients under 15 years old who had been admitted in respiratory intensive care unit(RICU) from January, 1983 to December 1984. 1) The mortality rates in 1983 and 1984 were 8.7% and 5.9% and the number of patients was increased but the mortality rate was decreased than before 1983. 2) the mortality was significantly high in neonates and infants but gradually decreased with age. 3) Of 730 patients, 30 of 437 male and 24 of 393 female patients were dead. 4) Of 730 patients, 667 were the patients of chest surgery and 56 were the patients of pediatric surgery but 7 were the patients in the departments other than in chest surgery or pediatric surgery. The Mortalities in chest surgery, pediatric surgery and other surgical departments were 5.4%(36 patients) 26.8%(15 patients) and 37.5%(3 patients). 5) Of 667 patients in department of chest surgery, 640 were the patients with the congenital heart disease and the mortality was 5.2%(33 patients) but 12 were the patients with acquired heart disease and the mortality was 16.7%(2 patients). Of 667 patients, 15 were the patients with noncardiac disease and mortality was 0.1%(1 patients). 6) The duration of the mechanical ventilation in RIDU patients, was 54.5 hours on the average and in the patients of mechanical ventilation more than 72 hours, the mortality was significantly increased compared with the patients of mechanical ventilation less than 72 hours. 7) The major causes of the death in RICU patients under 15 yearts old were LCOS(low cardiac output syndrome) and sepsis.