The first case of γ-heavy chain disease described in Spain is here reported. The patient, a 36-year-old woman, presented fever, enlarged regional lymph nodes, and hepatosplenomegaly, without bone marrow abnormalities but with lymphopenia. Serum electrophoresis did not disclose any M-component. The abnormal γ-chain protein had an α₂-globulin mobility and was immunochemically related to the Fc fragment. It belonged to the IgG 4 subclass, its molecular weight was about 60,000. Proteinuria was minimal but the electrophoresis of concentrated urine showed a homogeneous peak of γ-globulin mobility constituted by the γ-chain fragment. Biopsy of an axillary lymph node disclosed features of immunoblastic sarcoma. The course was malignant, resulting in death in 8 months.