Primary isolated bone marrow disease as a presenting feature of lymphoma is very rare. We describe the case of a Chinese with isolated bone marrow small B-cell lymphoma as a first manifestation. A 55-year old woman was admitted to our hospital with fever. Her peripheral blood smear and laboratory findings were suggestive of bicytopenia. Bone marrow specimen showed diffusely distributed small-sized lymphocytes. Combined with immunophenotypic and chromosomal analysis, a diagnosis of primary bone marrow B-cell non-Hodgkin's lymphoma was made. The patient was treated with R-CHOP (rituximab and cyclophosphamide, epirubicin, vindesine, and prednisone) regimen for six cycles. She had complete remission and is still alive without relapse. We concluded that primary bone marrow mature small B-cell lymphoma is a rare but distinctive subtype of lymphoma. The prognosis for this entity is poor but rituximab-based treatment is promising for improving its outcomes.