Objective: Chronic adenotonsillar hypertrophy is the most common etiologic reason for upper airway obstruction in childhood and has been found to be associated with a variety of psychiatric disorders and poor quality of life. In the present study, we investigated the impact of adenotonsillectomy on attention deficit hyperactivity disorder symptoms, sleep problems, and quality of life in children with chronic adenotonsillar hypertrophy. Methods: The parents of children with chronic adenotonsillar hypertrophy filled out the Conners’s Parent Rating Scale-Revised Short form (CPRS-RS), Children’s Sleep Habits Questionnaire (CSHQ), and Pediatric Quality of Life Inventory, Parent version (PedsQL-P) before and six months after adenotonsillectomy. Results: A total of 64 children were included in the study (mean age = 6.8 ± 2.4 years; boy:girl ratio= 1). The mean attention deficit hyperactivity disorder index and oppositionality subdomain scores of the CPRS-RS and all of the CSHQ subdomain scores (bedtime resistance, sleep-onset delay, sleep anxiety, night waking, parasomnias, sleep-disordered breathing, and daytime sleepiness) except for sleep duration significantly decreased after adenotonsillectomy (p < 0.05). The PedsQL-P total score and both PedsQL-P physical health and psychosocial health subdomain scores were significantly higher at six months after adenotonsillectomy (p < 0.001). Conclusions: Child and adolescent psychiatrists should check the symptoms of chronic adenotonsillar hypertrophy to identify children with chronic adenotonsillar hypertrophy who suffer from sleep disturbance, attention deficit hyperactivity disorder symptoms, and oppositionality. Adenotonsillectomy seems to be beneficial for coexisting attention deficit hyperactivity disorder and sleep disorder symptoms and quality of life in these children.