Abstract： hypogammaglobulinemia of infancy;follow-upBackground Hypogammaglobulinemia is common in infant humoral immunodeficiencies and has complicated causes and outcomes.We aimed to determine the clinical manifestations,immunological changes and outcomes of Shanghai infants with hypogammaglobulinemia.Methods Patients under 2 years old,having one or more warning signs of primary immunodeficiency disorders,serum immunoglobulin levels below the lower limit of reference range per age,and with normal numbers for lymphocyte subsets,were analyzed and followed up for 2 to 3 years.Results A total of 91 children (male-to-female ratio:2.25:1) participated in the study.Initial clinical presentation was recurrent upper respiratory tract infection (46%),invasive infection (3%),atopic disease (32%).IgA reduction (77%) was prevalent;34% patients had more than one isotype reduced.During follow-up,51 of 62 patients (82.25%) had immunoglobulins normalized at the age between 12-48 months; these were diagnosed as transient hypogammaglobulinemia of infancy (THI).Long-term follow-up may reveal a diagnosis for the remaining 11 infants with persistent lower immunoglobulin levels,who did not have antibody titers measured.Earlier onset was correlated with higher rates of normalization.More patients were diagnosed with isolated hypogammaglobulinemia in 2006 compared with the previous 4 years (2002-2005).Conclusions The awareness of immunodeficiency among pediatricians has been greatly improved.Recurrent otitis media was not a major infection in our patients.THI is a relatively common condition associated with infant hypogammaglobulinemia.In the absence of specific antibody titers,the diagnosis of THI can be confirmed retrospectively with lg levels normalized in follow-up visits.Therefore,long-term follow-up and frequent re-evaluation of these patients are necessary to distinguish them from true primary immunodeficiency.