e19550 Background: Rituximab, a monoclonal antibodies designed against a CD20 glycoprotein expressing lymphocytes. This drug has been implemented in the treatment of B cell lymphoproliferative disorders as well as non-neoplastic disease such as autoimmune thrombocytopenia, immune hemolytic anemia and other disorders related to overexpression of antibodies against the host. Rituximab doccumented side effects such as allergic infusion reaction, tumor lysis syndrome and lymphodepletion are frequently seen. However, transient neutropenia is less common. This abstract reports recurrent and long standing pancytopenia that had adverse deadly sequellae. Methods: This abstract report five cases of recurrent and prolonged pancytopenia not related to concomittant cytotoxic effect of chemotherapy. It is a retrospective analysis of treating three hundred patients in the last six years. Results: Five patient sustained recurrent prolonged pancytopenia post rituximab therapy. Two females (age 33 and 55) and three males (age 40, 50 and sixty). Mean age is 46 years. All developed sever neutropenia as well as pancytopenia. One patient, a female( age 33) had rheumatoid arthritis and the rest had non hodgekin's lymphoma. This female patient died due to sever long standing neutropenia/pancytopenia with pneumonia. All patients had bone marrow biopsy for evaluation. Decreased cellularity below 40% seen. None of the patients with non hodgkin's lymphoma had bone marrow involvement. The four patients are still in remmission. Conclusions: In conclusion the etiology of sustained neutropenia and pancytopenia is not understood. Whether, bone marrow aplasia, dysplasia or other immune mechanism exist need more study. No significant financial relationships to disclose.