Forkhead box P1 protein is a transcription factor involved in cell signaling and regulation of gene expression and is essential for B-cell development. Forkhead box P1 overexpression has been associated with a worsened prognosis in some B-cell lymphomas. However, little is known about the clinicopathologic significance of forkhead box P1 in T-cell malignancies. In this study, immunohistochemistry for forkhead box P1 was performed in peripheral T-cell lymphoma, not otherwise specified, cases (n = 41), which were then divided into lower (n = 15) and higher (n = 26) forkhead box P1 expressers. Results of real-time quantitative reverse transcriptase polymerase chain reaction for forkhead box P1 messenger RNA supported the data on immunohistochemical forkhead box P1 expression. Forkhead box P1 overexpression in lymphoma cells was inversely associated with proliferation activity as evaluated by Ki-67 expression. Double immunostain for forkhead box P1 and a T-cell marker in normal lymph nodes showed forkhead box P1 signals in many of nonneoplastic T cells. Prognostic analysis showed that forkhead box P1 overexpression was associated with an improved overall survival of the patients with peripheral T-cell lymphoma, not otherwise specified, and was independent of the International Prognostic Index in multivariate analysis. Forkhead box P1 overexpression may be associated with less activated phenotype of the tumors and with a better prognosis in patients with peripheral T-cell lymphoma, not otherwise specified. The clinicopathologic significance of forkhead box P1 overexpression in peripheral T-cell lymphoma, not otherwise specified, may be different from that in B-cell lymphomas.