nm23-H1 was originally identified as a protein that is expressed at a lower than usual level in metastatic cancer cells. The nm23 genes play critical roles in cellular proliferation, differentiation, oncogenesis, and tumor metastasis. Peripheral T-cell lymphoma (PTCL) is relatively rare, accounting for only 10% to 15% of non-Hodgkin's lymphomas. We examined whether nm23-H1 is a prognostic factor of peripheral T-cell lymphoma, not otherwise specified (PTCL-NOS). PTCL is more aggressive and has a poorer prognosis than diffuse large B-cell lymphoma. nm23-H1 was positive in 44.1% of PTCL-NOS patients. nm23-H1 expression was not correlated with age, performance status (PS), lactate dehydrogenase (LDH) level, or stage. The nm23-H1-positive group had significantly shorter overall survival (OS). OS was significantly shorter in patients with the following clinicopathologic features: age ＞ 60 years, PS of 2-4, LDH ＞ normal, bone marrow involvement, or nm23-H1-positive lymphoma. The nm23-H1 protein may be an important prognostic factor in PTCL-NOS. Because our results suggest that nm23-HI is produced by lymphoma cells, we expect to see the development of new treatments targeting nm23 overexpression.