Rituximab, an anti-CD20 chimeric antibody, is the first monoclonal agent to be used in the therapy of cancer. It has been hailed as one of the most important therapeutic developments of the decade. While transient peripheral B cell depletion is common after rituximab therapy, immunoglobulin levels are generally not affected. This is because CD20 is expressed on pre-B and mature B lymphocytes but not on stem cells or plasma cells. Two adult patients with pre-existing primary antibody deficiency who presented with recurrent infections immediately following rituximab use for the treatment of refractory idiopathic thrombocytopenic purpura (ITP) are described. Both were previously treated with various immunosuppressive agents without any notable infective problems. However, a few weeks after treatment with rituximab, these patients presented with clinically significant immunodeficiency requiring intravenous immunoglobulin replacement therapy. This striking temporal relationship between rituximab administration and onset of infections suggests that rituximab has accelerated the presentation of immune deficiency in these patients. Increased vigilance around the use of newer immunomodulatory agents such as rituximab is recommended.