Sero-mucous otitis (SMO) is particularly common in young children and a number of epidemiological studies exist suggesting the existence of a link between SMO and immediate allergy (allergic rhinitis in particular). However, studies of the composition of effusion fluid, middle ear mucosa and the effects of allergenic stimulation in the middle ear cavity lead to the virtually certain conclusion that SMO cannot be considered in itself as an allergic disorder. In contrast, prolonged tubal dysfunction, as may occur in certain cases of allergic rhinitis, is entirely capable of explaining the persistence of SMO, without being its actual direct cause. It is thus now reasonable to consider that SMO is a complication of allergic disorders, allergy being merely a cofactor associated with other etiological factors, e.g. viral or bacterial infections. In addition, the results of recent studies strongly suggest that SMO, or at least certain types, could well be the presentation of semi-delayed and/or delayed hypersensitivity reactions.