Background Childhood adenoid hypertrophy (AH) is common and is often associated with allergic asthma, resulting in complications like obstructive sleep apnea syndrome (OSAS). Management of the disease and its complications is often challenging. Case presentation We report here a case of a 10-year-old boy who suffered from severe allergic asthma and rhinitis and was treated with omalizumab. Before the treatment, the childhood asthma control test (C-ACT, 14), visal analog scale (VAS, 7) and lung function (mild obstructive ventilation dysfunction and moderate to severe dysfunction in ventilation in small airway) were seriously affected. Polysomnography showed OSAS (apnea hypopnea index, AHI, 6.4), low hypooxia saturation (lowest pulse oxygen saturation, LoSpO2, 70%), and adenoid hypertrophy (at grade III). After treating with omalizumab for 4 weeks (once treatment), the ventilation function, symptoms of asthma and allergic rhinitis (C-ACT, 24; VAS, 2), and OSAS (AHI: 1.8 and LoSpO2: 92.6%) were all improved, and the adenoids size was also significantly reduced to grade II. And during the following 3 times of treatment, the allergic symptoms continued improving, and the size of adenoid was reduced to grade I. Even 6.5 months after cessation of omalizumab, the size of adenoid remained at grade I. Conclusion This is the first documented case that childhood adenoid hypertrophy can be significantly improved by omalizumab. Keywords: Immunoglobulin E, Allergic rhinitis, Allergic asthma, Obstructive sleep apnea syndrome, Children