We describe the clinicopathological features of 20 patients with T/natural killer (NK)-cell lymphoma-associated hemophagocytic syndrome (T/NK-LAHS). These patients were categorized into 2 groups according to the onset of hemophagocytic syndrome (HPS). Group 1 developed HPS during the clinical course, typically at the terminal phase of the disease. This group consisted of 7 patients with extranodal lymphoma arising in the nasal cavity, paranasal cavity, tonsils, or skin at presentation. In 5 of these patients, the preferred diagnosis was nasal and nasal-type NK/T-cell lymphoma, whereas the disease diagnoses in the remaining 2 patients were peripheral T-cell lymphoma of unspecified type and angioimmunoblastic T-cell lymphoma, respectively. Group 2 consisted of 13 patients whose disease corresponded to so-called malignant histiocytosis-like lymphoma, which is characterized by HPS at the initial presentation and the infiltration of the liver, spleen, and/or bone marrow without tumor formation. Nine of these 13 cases were found to have common histopathological features: CD56+, Epstein-Barr virus positivity, cytotoxic molecules, and nasal-type NK/T-cell lymphoma. The very poor prognosis of T/NK-LAHS may be partly explained by the finding that nasal and nasal-type NK/T-cell lymphoma, which is resistant to standard chemotherapy, made up the highest percentage (70%) of the cases.