Serum sickness may occur in patients treated with chimeric monoclonal antibody. Rituximab, an anti-CD20 chimeric monoclonal antibody, is used with increasing frequency in chronic immune thrombocytopenic purpura (ITP). Rituximab is relatively safe; however, serum sickness is reported in 1-20% of patients, more commonly among those with autoimmune conditions. We describe a case of serum sickness in a patient with ITP and review the literature of rituximab-induced serum sickness.