Arsenic compounds have been used in traditional medicine for several centuries. KML001 (sodium metaarsenite; NaAsO2) is an orally bio-available arsenic compound with potential anti-cancer activity. However, the effect of KML001 has not been studied in lymphoid neoplasms. The aim of this study is to evaluate the anti-proliferative effect of KML001 in non-Hodgkin's lymphoma and to compare its efficacy with As2O3. KML001 inhibited cellular proliferation in all tested lymphoma cell lines as well as JurkatR cells (adriamycin-resistant Jurkat cells) in a dose-dependent manner, while As2O3 was not effective. Cell cycle regulatory protein studies have suggested that KML001 induces G1 arrest via p27-induced inhibition of the kinase activities of CDK2, 4, and 6. Treatment of KML001 induced apoptosis in Jurkat and JurkatR cells. The apoptotic process was associated with down-regulation of Bcl-2 (antiapoptotic molecule), up-regulation of Bax (proapoptotic molecule), and inhibition of caspase-3, -8, and -9. In addition, cell signaling including the STAT, PI3K/Akt, MAPK, and NF-κB signal pathways were inhibited in KML001-treated Jurkat and JurkatR cells. Furthermore, targeting the telomere by KML001 was observed in the Jurkat and JurkatR cells. The In vivo anti-tumoral activity of KML001 was confirmed in a xenograft murine model. Interestingly, partial responses were seen in two lymphoma patients treated with 10 mg/day (follicular lymphoma for 16 weeks and mantle cell lymphoma for 24 weeks) without severe toxicities. These findings suggest that KML001 may be a candidate agent for the treatment of de novo, refractory, and relapsed non-Hodgkin's lymphoma patients.