Otitis media is one of the most common childhood infections and may result from a variety of underlying problems. Suspicion of immunodeficiency should increase when ear infections are frequent; suppurative; unresponsive to antibiotics; caused by unusual organisms; or seen in the context of other frequent infections, severe eczema, or failure to thrive. Humoral immune deficiencies, particularly with an inability to make antibody to encapsulated organisms, are the immunodeficiencies most likely to cause increased otitis media. Immune system evaluation should concentrate on humoral immunodeficiency disorders, but the presenting history and physical findings also should be considered when designing the work-up. Treating the underlying immune deficiency is usually necessary to adequately control the ear infections.