Objective:To study the prevalence of vitamin B6 deficiency in common variable immunodeficiency and the impact of vitamin B6 supplementation on immune function in the disorder.Design:Open, non-blinded.Setting:Medical School Hannover, Hannover, Germany.Subjects:Plasma vitamin B6 concentrations were measured in all the 54 common variable immunodeficiency (CVID) patients visiting our outpatients' clinics in 2005.Interventions:The 17 patients with a decreased vitamin B6 concentration were recommended to take 50 mg of vitamin B6/day for 3 months. Then, vitamin B6 concentrations, absolute number of lymphocyte populations and immunoglobulin concentrations were controlled.Results:Vitamin B6 concentrations were reduced in 17/54 patients. All 11/17 patients following our advice to substitute vitamin B6 had normal vitamin B6 plasma concentrations 3 months later. In parallel, the number of CD4+ T cells significantly increased. In contrast, concentrations of serum immunoglobulins were not improved.Conclusions:Vitamin B6 deficiency is common in CVID. The vitamin deficiency is not the cause of CVID and vitamin supplementation does not relieve humoral immunodeficiency. Nevertheless, vitamin B6 should be measured in CVID to avoid possible long-term complications of its deficiency.