The study aims to explore the inflammatory cytokines and oxidative stress in children with obstructive sleep apnea syndrome (OSAS) triggered by adenoids and/or tonsillar hypertrophy and their changes after adenotonsillectomy (AT) and to investigate the associated behavioral disorders in OSAS, before and after AT. Thirty patients with OSAS and 20 healthy children, aged 3 - 13 years were included in the study. According to apnea-hypopnea index (AHI), OSAS children were classified into 3 groups: mild (n = 19), moderate (n = 5), and severe OSAS (n = 6). Erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), interleukin (IL)-6, tumor necrosis factor (TNF)-α, malondialdehyde (MDA) and antioxidant enzymes activities were assessed in serum, preoperative and 6 weeks after AT. TNF-α, IL-6 and malondialdehyde levels were also estimated in adenoid and tonsils tissues. A Pediatric Sleep Questionnaire was completed by the parents before and after AT. As a result of the study, we obtained the following results: TNF-α, IL-6 and malondialdehyde evaluated preoperative increased in serum and tissues in OSAS, especially in severe disease compared to mild and moderate forms. Six weeks after AT, AHI diminished significantly in OSAS, as well as the inflammatory markers and malondialdehyde, in parallel with significant improvement of antioxidant enzymes activities. Daytime sleepiness, hyperactivity and attention deficit in OSAS, even in mild disease were present, with significant improvements of obstructive symptoms after AT. We conclude that OSAS caused by adenoids and/or tonsillar hypertrophy led to changes in the blood parameters, with significant improvement after AT. Postoperatively, a significant improvement in sleep quality and behavior in OSAS patients was also observed.