Upon enrollment in the study, patients will have a blood sample collected for genotyping of the FCGR3A gene (immunoglobulin Fc G receptor IIIa), the ABCB1 gene (ATP Binding Cassette Beta 1; also called MDR1), and other candidate genes. Patients will be treated with R-CHOP for six cycles, which is standard therapy for advanced stage DLCL. Response will be monitored by an FDG-PET scan performed after 2 cycles of R-CHOP and restaging exams performed upon completion of chemotherapy. Gene polymorphisms will be analyzed to establish which polymorphisms predict response to R-CHOP.