We report a case of natural killer (NK) cell leukemia with unusual biological features in a 65-year-old woman with a 20-year history of rheumatoid arthritis. She presented with neutropenia, thrombocytopenia, splenomegaly and bone marrow infiltration. Immunophenotyping (CD2+ CD3- CD4- CD5- CD8- CD16+ CD56-) confirmed NK cell leukemia. Her neutropenia and thrombocytopenia resolved following splenectomy and she has remained well with stable disease for 12 months on oral low-dose methotrexate. In contrast to all previous reports, in this instance the phenotype of large granular lymphocytic (LGL) leukemia occurring in the context of rheumatoid arthritis was NK-cell rather than T-cell. Furthermore, the clinical course was indolent, whereas, all prior literature reports have described a very aggressive clinical course for this disorder with a median survival of just 3.5 months. This case illustrates previously unrecognized heterogeneity in the natural history of this disorder.