A 10-year prospective cohort study followed the evolution of antibodies in children less than 4 years of age with hypogammaglobulinemia. Three patterns were identified: in group 1, immunoglobulins and antibody production normalized;iin group 2, patients continued to have low IgG levels; and in group 3, IgG levels normalized but antibody levels were transient. Statistical analysis showed that invasive infection or low tetanus antibodies at presentation were associated with the development of significant humoral immunodeficiency.