Adolescence is a critical phase for achieving human potential, serving as the foundation for later health. In 2010, the major causes of hospital admissions and deaths in Thai adolescents were related to preventable causes, specifically engagement in high-risk behaviors such as unprotected sexual intercourse, substance use and unsafe driving. We retrieved data from 1,761,667 adolescent (10–17 years) hospital admissions and 6362 deaths between 2015 and 2019 from the National Health Security Office database. Trends of hospital admissions and deaths, length of stay and medical expenses by sex, age: early (10–13 years) and middle adolescents (14–17 years), and geographical regions were analyzed by ICD-10 disease group and single diagnosis. Hospital admissions increased in relation to the same age population from 2015 to 2019. Trends of the top 3 diagnoses shifted between disease groups from 2015 to 2019. Pregnancy retreated from the first (17.8%) to the third rank (12.2%), and arthropod-borne viral fevers advanced from the third (13.1%) to the first rank (17.1%). Injury and poisoning remained at the second rank (14.5–14.4%). Females were admitted more than males, but males had significantly longer hospital stays. Early adolescents were admitted less than middle adolescents and their hospital stay was significantly shorter. Trends of the top 3 diagnoses for deaths by disease groups remained stable: injury and poisoning (41.2%), neoplasms (10.2%), and respiratory infections (9.5%). The average direct health care cost utilized on adolescent health care was 3813 million Baht (115.54 million US Dollars) per year. Considering the top 3 disease groups, injury and poisoning had the highest average cost per hospital admission and net cost per year consuming 26.4% of the total cost. Our study highlights the cause of hospital admissions and deaths in Thai adolescents, which are mainly preventable. Adolescent health care will improve with more investment in prevention through policy, service, and education reform.