Mature natural killer (NK)-cell neoplasms include extranodal NK/T cell lymphoma, nasal type (ENKL), aggressive NK-cell leukemia (ANKL) and chronic lymphoproliferative disorders of NK cells (CLPD-NK). Granulysin, a cytolytic granule protein, is expressed in cytotoxic T cells and NK cells, and is found in the sera as well, and functions as a cytotoxic and proinflammatory protein. Cytolytic proteins, such as granzyme B and perforin, have been shown to play crucial pathophysiological roles in NK/T cell neoplasms and have also been utilized for diagnostic purposes. Granulysin in NK-cell proliferative disorders, however, has yet to be fully analyzed. To elucidate the clinical relevance of granulysin in mature NK-cell neoplasms, we measured serum granulysin and analyzed cytolytic molecules immunohistologically. The median concentrations of serum granulysin were 39.0, 2.85, 2.8 and 1.35 ng/ml in ANKL, ENKL, CLPD-NK and healthy subjects, respectively (P < 0.01). Serum granulysin was significantly elevated in patients with ANKL compared with the levels in ENKL (P = 0.006) and CLPD-NK (P = 0.037). Furthermore, serum granulysin was correlated with whole-blood EBV viral load in ENKL and ANKL (P = 0.005) and was significantly reduced after treatment. Different expression patterns of cytolytic granule proteins were observed among the mature NK-cell neoplasms. Granulysin is closely associated with the characteristics of NK-cell neoplasms and serum granulysin may serve as a novel biomarker for these disorders.