Backgrounds and aims: Given problematic Internet usage's (PIU) negative impact on individual health, this study evaluates how adverse childhood experiences (ACEs) affect young adults' PIU and the possible underlying mechanism of the "feeling of loneliness" (FOL) trajectory. Methods: Analyzing a retrospective cohort sample from the Taiwan Youth Project, 2,393 adolescents were interviewed from the average ages of 14-28. We constructed ACE in 2000 using six categories (e.g., abuse and low family socioeconomic status) and 5-item PIU in 2017 from Chen's Internet Addiction Scale. FOL trajectories measured eight times, at average ages 14, 16, 17, 18, 20, 22, 25, 28 years-old. Results: Overall, 12.65% of the participants did not have ACEs, and 12.78% exhibited PIU. FOL trajectory analyses yielded three groups: "constant low" (reference group: 53.25%); "moderate decline" (36.81%); and "increasing" (9.94%). Regression models showed a dose-response association between ACE and young adults' PIU (adjusted odds ratio 5 1.12; 95% confidence interval [CI] 5 1.02-1.23) and the two risky loneliness groups (moderate decline: relative risk ratio [RRR] 5 1.42, 95% CI 5 1.32-1.54; increasing: RRR 5 1.52, 95% CI 5 1.37-1.71). Structural equation modeling further found that ACEs increase young adults' risk of being in the increasing group, and consequently, the risk of PIU. Discussion and conclusions: We demonstrated that ACE may be associated with 1) adults' PIU, 2) FOL from adolescence to emerging adulthood, and 3) young adults' PIU through its association with FOL trajectories.  Kulcsszavak: adverse childhood experiences (ACE), problematic internet use (PIU), loneliness trajectories, young adults, longitudinal study