We conducted a phase 1 trial evaluating the oral nucleoside analog clofarabine in patients with relapsed/refractory non-Hodgkin lymphoma. Patients were treated once daily on days 1 through 21 of a 28-day cycle for a maximum of six cycles. The study was conducted with a 3 + 3 design with 10 additional patients treated at the recommended phase 2 dose. Thirty patients were enrolled including indolent B-cell lymphomas (n = 21), mantle cell lymphoma (n = 6) and diffuse large B-cell lymphoma (n = 3). The primary toxicities were hematologic including grade 3-4 neutropenia (53%) and thrombocytopenia (27%). Three milligrams was determined to be the recommended phase 2 dose. Tumor volume was reduced in 70% of patients, and the overall response rate was 47% including 27% complete remissions. Responses were seen in indolent B-cell lymphomas and mantle cell lymphoma. At a median follow-up of 17 months, 68% of responding patients remain in ongoing remission. Oral clofarabine was well tolerated with encouraging efficacy in indolent B-cell lymphomas and mantle cell lymphomas, warranting further investigation.