A 68-year-old woman was referred to Oami Hospital for severe leukocytopenia. Laboratory data revealed severe leukocytopenia (0.8x10(9)/l), severe neutropenia (0.22x10(9)/l) and IgM-kappa monoclonal gammopathy. Granular lymphocytes accounted for more than 90% of leukocytes. Surface marker analysis revealed that these lymphocytes were CD3+, CD4-, CD8+ and CD16+, and monoclonal rearrangement of T-cell receptor genes was seen on Southern blot hybridization analysis. She was diagnosed with T-cell lineage granular lymphocyte proliferative disorder (T-GLPD). During 13-year follow-up without granulocyte-colony stimulating factor (G-CSF) administration, except for one hospitalization because of bacterial pneumonia, she has not experienced severe infection, despite severe neutropenia. T-GLPD should be kept in mind as a cause of leukocytopenia with or without monoclonal gammopathy, and surface marker or Southern blot hybridization analysis should be considered for determining the diagnosis. There are many reports about Caucasian patients with T-GLPD suffering from recurrent infections and dying of severe infection, but in Japanese T-GLPD patients, severe infection is very rare, as shown in the present case report.