Primary immunodeficiencies are defined as genetically determined functional and/or quantitative abnormalities in one or more of the components of the immune system. Immunodeficiency and arthritis can be related, although the mechanisms are not always clear. Different causes for immunodeficiency can secondarily be found in patients with arthritis; on the other hand, arthritis can be a manifestation of primary immunodeficiency. Arthritis occurs chiefly in humoral primary immunodeficiencies, namely in X-linked agammaglobulinemia and common variable immunodeficiency, and may be one of the warning signs for primary immunodeficiency. We report a case of arthritis as the presenting feature of X-linked agammaglobulinemia. In X-linked agammaglobulinemia, arthritis may be a consequence of infection, most notably by Mycoplasma, or of immune dysfunction itself. In children, and occasionally in young adults, a combination of arthritis and hypogammaglobulinemia should suggest primary immunodeficiency, although other causes of hypogammaglobulinemia must be excluded. Physicians evaluating patients with arthritis should be aware of this fact so that an early diagnosis can be pursued as it is of extreme importance in the optimal management and prognosis of these patients.