Internet addiction (IA) has become a serious problem of modern addictology, especially among the younger generation. There is a sufficient number of works devoted to the comorbidity of IA and other mental and behavioral disorders, but the studies of the IAcharacteristics in mentally ill patients are of a single nature. The goal of this study was to evaluate the characteristics of the IA behavior in children and adolescents with various mental disorders. The main group consisted of 71 adolescents, 28 boys and 43 girls, aged 11-18 years, treated in a psychiatric hospital with diagnoses Organic disorders (F 06, F 07)—35 subjects, Behavior disorders (F 91, F 92) -16, Neurotic, stress-related disorders (F 41, F 43) -10 and schizophrenia, schizotypic disorder (F 20, F21) -10. The control group consisted of 49 healthy adolescents, 24 boys, 25 girls, aged 11-18 years. The following methods were used: Сlinical-anamnestic map, Participant questionnaire for assessing the parameters of IA behavior, of Chen Internet addiction scale (CIAS—in the adaptation of V.L. Malygin and K.A. Feklisov) to assess the severity of FM. It is shown that among teenagers with mental disordersIA occurs significantly more often (more than ¼) than in the control group, the same for females and males. Mentally ill adolescents used the Internet significantly more hours per week, used social networks more per day, had a larger overall score on the scale of CIAS. It is established from gender characteristics that girls used social networks more often than boys. The risk of IA was higher in users of gaming communities. IA was more often observed in patients with organic disorders. The tendency to IA behavior was more frequent in patients with behavioral disorders and schizophrenia.