Objective:To characterize the longitudinal and dynamic high-density lipoprotein (HDL) trajectories in critically ill children and explore their correlation with clinical outcomes.Methods:Retrospective cohort study.All critically ill children admitted to the Pediatric Intensive Care Unit (PICU) of West China Hospital, Sichuan University from January 1, 2015 to October 1, 2020 were included in this retrospective study.Group-based trajectory modeling (GBTM) was applied to characterize the HDL trajectories in days 0-6 post-PICU admission and develop HDL trajectory groups.The in-hospital mortality rate was reported as frequency (%) and then compared by the Chi-square test or Fisher′s exact test between HDL trajectory groups.The length of stay (LOS) in the PICU was described by M( Q1, Q3), and its difference between HDL trajectory groups was evaluated by the Kruskal Wallis test.Logistic regression and multiple linear regression were used to determine the correlation between HDL trajectories and clinical outcomes.The primary outcome was in-hospital mortality rate, and the secondary outcome was LOS in the PICU. Results:A total of 4 384 critically ill children were ultimately enrolled in the study, and 6 HDL trajectory groups were developed based on GBTM analyses: group 1 (758 cases), the lowest HDL group; group 2 (1 413 cases), the low HDL group; group 3 (74 cases), the low-to-high HDL group; group 4 (621 cases), the medium HDL group; group 5 (1 371 cases), the high HDL group; and group 6 (147 cases), the highest HDL group.Logistic regression analysis showed that compared with critically ill children in group 1, those belonging to groups 2, 3, 4, 5, and 6 were at lower risks of in-hospital mortality with odds ratio ( OR): 0.475, 95%confidence interval ( CI): 0.352-0.641, P<0.001; OR: 0.093, 95% CI: 0.013-0.679, P=0.019; OR: 0.322, 95% CI: 0.208-0.479, P<0.001; OR: 0.263, 95% CI: 0.185-0.374, P<0.001, and OR: 0.142, 95% CI: 0.044-0.454, P=0.001, respectively.Multiple linear regression analysis revealed that compared with critically ill children in group 1, those belonging to groups 4, 5, and 6 had the trend of shorter LOS in PICU, and the β value and 95% CI were β: -4.332, 95% CI: -5.238- -3.426, P<0.001; β: -3.053, 95% CI: -3.809--2.297, P<0.001; β: -6.281, 95% CI: -7.842--4.721, P<0.001, respectively. Conclusions:The dynamic HDL trajectories during 0-6 days after PICU admission are associated with in-hospital mortality rate of critically ill children.The HDL trajectory at a persistently low level is associated with higher mortality, while the HDL trajectory at a persistently high level or with the trend from a low level rising to a high level shows a lower risk of mortality.It is suggested that the HDL trajectory model may become an indicator to predict the condition and prognosis of critically ill children.