Neoplasms putatively originating from precursor and mature natural killer (NK) cells are rare, and their clinical features are unclear. A nationwide survey was performed in Japan to clarify the clinical features of these neoplasms diagnosed between 1994 and 1998, and data for 237 patients who met the criteria for putative NK cell-lineage neoplasms were analyzed. Among them, 11 had myeloid/NK-cell precursor acute leukemia, 15 blastic NK-cell lymphoma, 21 precursor NK-cell acute lymphoblastic leukemia, 22 aggressive NK-cell leukemia/lymphoma, 149 nasal-type NK-cell lymphoma (123 nasal and 26 extranasal) and 19 chronic NK lymphocytosis. The median overall survival time of patients with aggressive NK-cell leukemia/lymphoma was 2 months, which for chronic NK lymphocytosis was more than 8 years, and that for the other types of NK-cell neoplasms was between 6 and 22 months. Nasal NK-cell lymphoma and extranasal NK-cell lymphoma share the same histology. The age of affliction was the same, but the sex was different with males predominantly having nasal NK-cell lymphoma and females extranasal NK-cell lymphoma. Patients with extranasal NK-cell lymphoma had the tendency to exhibit a more advanced state of disease, with significantly higher International Prognostic Index and LDH levels, and significantly lower hemoglobin and platelet levels. The overall survival, however, did not differ significantly. Precursor NK-cell acute lymphoblastic leukemia and blastic NK-cell lymphoma were arbitrarily defined by the presence or absence of 30% or more of blastic cells in the bone marrow or peripheral blood, but there were no significant differences for affected age, gender, involved sites or prognosis. Aggressive NK-cell leukemia/lymphoma and extranasal NK-cell lymphoma were arbitrarily defined by the presence or absence of 30% or more of large granular lymphocytes in the bone marrow or peripheral blood and it is possible that aggressive NK-cell leukemia/lymphoma is a leukemic phase of extranasal NK-cell lymphoma. The incidence of skin involvement, however, was significantly higher for extranasal NK-cell lymphoma, suggesting that the two diseases are different. In nasal NK-cell lymphoma, Epstein-Barr virus in tumor cells was detected in all patients tested, suggesting its causative role.