Lenalidomide was shown to have significant single-agent activity in relapsed aggressive non-Hodgkin's lymphoma (NHL). We conducted a phase I trial to establish the maximum tolerated dose of lenalidomide that could be combined with R-CHOP (rituximab-cyclophosphamide, doxorubicin, vincristine, and prednisone). Eligible patients were adults with newly diagnosed, untreated CD20 positive diffuse large cell or follicular grade III NHL. Patients received oral lenalidomide on days 1-10 with standard dose R-CHOP every 21 days. All patients received pegfilgrastim on day 2 of the cycle and aspirin prophylaxis. The lenalidomide dose levels tested were 15, 20 and 25 mg. A total of 24 patients were enrolled. The median age was 65 (35-82) years and 54% were over 60 years. Three patients received 15 mg, 3 received 20 mg and 18 received 25 mg of lenalidomide. No dose limiting toxicity was found, and 25 mg on days 1-10 is the recommended dose for phase II. The incidence of grade IV neutropenia and thrombocytopenia was 67% and 21%, respectively. Febrile neutropenia was rare (4%) and there were no toxic deaths. The overall response rate was 100% with a complete response rate of 77%. Lenalidomide at the dose of 25 mg/day administered on days 1 to 10 of 21-day cycle can be safely combined with R-CHOP in the initial chemotherapy of aggressive B-cell lymphoma.