Patients with congenital or acquired immunodeficiencies are at an increased risk to develop polyclonal or oligoclonal lymphoid malignancies. Some develop a lymphoproliferative disorder that can follow a clinically aggressive course and may represent a pre-malignant lesion. Although most of these lymphoproliferative disorders are of B-cell origin, T-cell or non-B-non-T-cell processes have also been observed. The pathogenesis is only partially understood.