Purpose: The standard treatment of patients with diffuse large B-cell lymphoma (DLBCL) is rituximab in combination with cyclophosphamide, doxorubicin, vincristine, and prednisone (R-CHOP). Lenalidomide, an immunomodulatory agent, has shown activity in DLBCL. This randomized phase III trial compared lenalidomide as maintenance therapy with placebo in elderly patients with DLBCL who achieved a complete response (CR) or partial response (PR) to R-CHOP induction. Methods: Patients with previously untreated DLBCL or other aggressive B-cell lymphoma were 60 to 80 years old, had CR or PR after six or eight cycles of R-CHOP, and were randomly assigned to lenalidomide maintenance 25 mg/d or placebo for 21 days of every 28-day cycle for 24 months. The primary end point was progression-free survival (PFS). Results: A total of 650 patients were randomly assigned. At the time of the primary analysis (December 2015), with a median follow-up of 39 months from random assignment, median PFS was not reached for lenalidomide maintenance versus 58.9 months for placebo (hazard ratio, 0.708; 95% CI, 0.537 to 0.933; P = .01). The result was consistent among analyzed subgroups (eg, male v female, age-adjusted International Prognostic Index 0 or 1 v 2 or 3, age younger than 70 v ≥ 70 years), response (PR v CR) after R-CHOP, and positron emission tomography status at assignment (negative v positive). With longer median follow-up of 52 months (October 2016), overall survival was similar between arms (hazard ratio, 1.218; 95% CI, 0.861 to 1.721; P = .26). Most common grade 3 or 4 adverse events associated with lenalidomide versus placebo maintenance were neutropenia (56% v 22%) and cutaneous reactions (5% v 1%), respectively. Conclusion: Lenalidomide maintenance for 24 months after obtaining a CR or PR to R-CHOP significantly prolonged PFS in elderly patients with DLBCL.