Although middle ear effusion is the most common of the chronic conditions encountered in pediatric practice, it frequently goes unrecognized, and relatively little is known about its epidemiology or its natural history. More importantly, the impact of middle ear effusion on the overall well-being of infants and children, and on their cognitive and language development, remains essentially unexplored. In part because of these gaps in our knowledge, and also because prospective, controlled treatment trials have not been reported, uncertainty and controversy exist concerning: 1) the necessity, in many instances, of treating middle ear effusion; 2) the choice of various treatment methods; and 3) the timing of those treatments that are undertaken. Finally, the widespread use of adenoidectomy to prevent middle ear inflammation is seriously lacking in supporting evidence. A few reported studies bear on some of these questions, but both their scope and the information they provide are limited. In order to resolve important prognostic and treatment issues concerning middle ear effusion during infancy and childhood, carefully designed epidemiologic and clinical studies of types not hitherto reported are urgently needed.