Back pain with sciatica is a common clinical syndrome that may be the expression of an uncommon pathological process. We describe a case of retroperitoneal lymphoma mimicking herniated lumbar disc. 62 year old male patient with a clinical picture of sciatica as a first manifestation of retroperitoneal lymphoma. His pain was not response to conventional therapy, and progressively worsened. We reevaluated him and found huge retroperitoneal lymphoma in his pelvic cavity. So we discuss how to identify the few who had extra attention. It is important to rule out occult pathology in patient with an atypical presentation. In assessing a patient with a low back pain, accurate evaluation require a logical, step by step method.