Low serum immunoglobulin E (IgE) (<2 kU/L) found during allergy investigations may be a marker for other immunodeficiency states, in particular common variable immunodeficiency. The latter is characterised by recurrent infections, mainly respiratory, resulting in structural lung damage making early diagnosis important. We looked through 4013 samples retrospectively over a 12-month period to identify samples with IgE <2 kU/L. We identified 74/4013 (1.84%) with serum IgE levels <2 kU/L. Only 20 samples had serum immunoglobulin quantification and serum electrophoresis requested. Three of these samples were from the same patient, 10/18 (56%) had one or more classes of immunoglobulin above/below reference range for age and two of these had new diagnosis of immunodeficiency. Serum IgE <2 kU/L can be a marker for hypogammaglobulinaemia or common variable immunodeficiency. As early diagnosis is important to reduce morbidity and mortality, very low serum IgE should trigger further investigation—that is, serum protein electrophoresis and immunoglobulin quantification.