This study has 3 parts. The purpose of Part 1 of this study is to assess the safety and efficacy of brentuximab vedotin in combination with RCHOP (rituximab, cyclophosphamide, doxorubicin, vincristine, and prednisone) (known as BV+RCHOP) in patients with DLBCL who have never been treated. Patients will be randomly assigned in a 1:1 ratio to receive RCHOP together with 1 of 2 doses of brentuximab vedotin. Patients will be tested to see if there is a difference in side effects between the 2 groups. The purpose of Part 2 of this study is to assess the safety and efficacy of brentuximab vedotin in combination with RCHP (rituximab, cyclophosphamide, doxorubicin, and prednisone) (known as BV+RCHP) in patients with CD30-positive DLBCL who have never been treated. Patients will be enrolled to receive RCHP together with 1.8mg/kg of brentuximab vedotin. The purpose of Part 3 of this study is to assess the safety and efficacy of BV+RCHP compared to standard RCHOP in patients with CD30-positive DLBCL that have never been treated. Patients will be randomly assigned in a 1:1 ratio to receive either BV+RCHP or RCHOP. Patients will be tested to see if there is a difference in side effects between the 2 groups.

In the first part of this study, patients in the 2 groups were tested to see if there was a difference in the response to treatment and whether there were differences in the side effects (unwanted effects). The second and third parts of the study are being done to see if there are any side effects (unwanted effects) of the higher dose of brentuximab vedotin when combined with a modified version of RCHOP that omits vincristine. The third part of the study is being done to see if there is a difference between BV+RCHP and RCHOP in the response to treatment.