We investigated the production of immunoglobulin (Ig) in six patients with common variable immunodeficiency and analyzed the courses of their levels of serum Ig for a period of 8–13 years. In all patients, levels of serum IgM, IgG and IgA were markedly low at the first examination, except for the IgM level in one patient. Improvement of serum Ig levels was observed in the patients in whom Ig production of non‐T cells at the first examination and Cμ gene expression had been detected to a slight degree, but serum Ig levels remained low in the patients in whom Ig production of non‐T cells and Cμ gene expression had not been detected. Our results suggest that some hypogammaglobulinemia in common variable immunodeficiency develops or improves w