A 59-year-old man with diffuse large B-cell lymphoma presenting as an anterior mediastinal mass underwent PET/CT after 2 cycles of chemotherapy. PET/CT shows moderately increased FDG uptake in a predominantly left prevascular mass, consistent with partially treated lymphoma. The left hemidiaphragm is elevated. PET shows asymmetric uptake along the right hemidiaphragm, which is hypothesized to represent compensatory increased workload related to contralateral left diaphragmatic paresis. The left diaphragmatic paresis is likely caused by phrenic nerve involvement by left prevascular mass.