Relevance. In the context of the pandemic, schoolchildren were transferred to a distance learning format. Forced isolation, prolonged stay at the computer, physical inactivity, high mental stress had an impact on their health. Of practical interest is the analysis of the frequency of chronic diseases during a pandemic in a cohort of adolescents. Purpose: to estimate the incidence of chronic diseases among adolescents during the COVID-19 pandemic. Materials and methods: a prospective, cohort, non-randomized study of adolescents (15-17 years old), (n=221): 116 girls, 105 boys were examined in accordance with the order of the Ministry of Health of the Russian Federation of August 10, 2017 N 514n “On the Procedure for Conducting Preventive Medical Examinations minors”, a comparison was made between the results of the survey in 2019 and 2021. Results. The frequency of mild protein-energy malnutrition increased from 15.1% (2019) to 19.6% (2021), p=0.02. The number of overweight children decreased in the general group from 14.1% to 10.1%, p=0.04, and in the group of boys from 17.2% to 10.6%, p=0.04. Obesity of the I-II degree in the observation groups did not have significant changes in the number of diagnosed cases, however, in the group of boys in 2021, obesity of the III degree was detected. According to the results of observation against the backdrop of a pandemic in adolescents, the frequency of chronic diseases has significantly increased: pathology of the organ of vision - by 1.4 times, pathology of the musculoskeletal system - by 2.8 times, pathology of the nervous system - by 7 times, gynecological diseases - by 1, 7 times. Conclusion. The overall frequency of detected pathology increased by 1.6 times, due to diseases of the eyes, blood system, musculoskeletal system, endocrine, nervous and reproductive systems. It is likely that physical inactivity and a high level of stress, causing a long-term hormonal imbalance, played a negative role during the period of covid restrictions. At present, thanks to the opening of rehabilitation departments providing specialized medical care within the framework of the compulsory medical insurance system, this category of patients throughout the Russian Federation will have the opportunity for personalized correction of the identified pathology.