Extranodal natural killer (NK)/T‐cell lymphoma, nasal type, and aggressive NK‐cell leukemia are rare, and their standard therapy has not been established. They are Epstein–Barr virus‐associated lymphoid malignancies, and tumor cells express P‐glycoprotein leading to multidrug resistance of the disease. Patients with stage IV, relapsed or refractory diseases have a dismal prognosis, with survival measured in months only. To develop an efficacious chemotherapeutic regimen, we conducted a dose‐escalation feasibility study of a new chemotherapeutic regimen, SMILE, comprising the steroid dexamethasone, methotrexate, ifosfamide, l‐asparaginase, and etoposide. The components of SMILE are multidrug resistance‐unrelated agents and etoposide. Etoposide shows both in vitro and in vivo efficacy for Epstein–Barr virus‐associated lymphoproliferative disorders. Eligible patients had newly diagnosed stage IV, relapsed or refractory diseases after first‐line chemotherapy, were 15–69 years of age, and had satisfactory performance scores (0–2). Four dose levels of methotrexate and etoposide were originally planned to be evaluated. At level 1, six patients with extranodal NK/T‐cell lymphoma, nasal type, were enrolled. Their disease status was newly diagnosed stage IV (n = 3), first relapse (n = 2), and primary refractory (n = 1). All of the first three patients developed dose‐limiting toxicities, and one of them died of sepsis with grade 4 neutropenia. A protocol revision stipulating early granulocyte colony‐stimulating factor administration was made. Two out of three additional patients developed dose‐limiting toxicities that were all manageable and transient. For the six enrolled patients, the overall response rate was 67% and the complete response rate was 50%. Although its safety and efficacy require further evaluation, we recommend a SMILE chemotherapy dose level of 1 for further clinical studies. (Cancer Sci 2008; 99: 1016–1020)