Additional Study Information: This study will test the standard R-CHOP21 against R-CHOP21 plus lenalidomide. The purpose is to see whether adding lenalidomide works better and is as safe as R-CHOP by itself. This study is only for patients with ABC type DLBCL who have not yet been treated. Lenalidomide is not approved for use in DLBCL. Its use in this disease is experimental. In this study, the experimental treatment is lenalidomide + R-CHOP21 x 6. This study will use a gene expression profile (GEP) test to see if a patient has the ABCtype. The results of this GEP test affect whether you may be treated on this study. Because the performance of this test has not been proven, this test is for investigational use only, and is still under development. This means the GEP test is considered an experimental test.