Mutations of the gene encoding the TNF receptor family member transmembrane activator and calcium-modulator and cyclophilin ligand interactor (TACI), TNFSRF13B, have recently been described in patients with common variable immunodeficiency (CVID). We report the case of a man with CVID in association with a heterozygous TACI gene mutation (C104R) who had a highly unusual, invasive, polyclonal CD8⁺ T-cell lymphoproliferation resulting in massive hepatosplenomegaly and causing renal impairment because of infiltration. Although lymphoproliferation is well described in CVID, the key features in this patient included the T-cell origin of the lymphoproliferation, its polyclonal nature, its infiltration into multiple organs, and the presence of the TACI gene mutation.