The aim of this study was to review the literature to evaluate the association between allergic rhinitis (AR) and obstructive sleep apnea syndrome (OSAS) in childhood. A PubMed literature search (January 1970 to February 2005) was conducted using the following key words: obstructive sleep apnea, allergic rhinitis, and mouth breathing. The retrieved articles were reviewed and the levels of evidence were assessed. AR affected approximately 40% of children and OSAS occurred in 2% of children. AR is a risk factor for OSAS because AR is associated with nasal obstruction, enlargement of tonsils and adenoids, and an elongated face, which, taken together, constitute a smaller upper airway size. Adequate treatment of AR is helpful to decrease the severity of OSAS and prevent emergence of an elongated face, which predispose for OSAS. There is convincing evidence that AR increases the risk of OSAS in children. Appropriate treatment of AR regularly could prevent the occurrence of OSAS and reduce the severity of existing OSAS.