An initial CT of a 59-year-old man with increasing back pain and weight loss showed lymphadenopathy in multiple nodal beds. A biopsy showed diffuse, large B-cell lymphoma (DLBCL). After initial chemotherapy, residual disease prompted an autologous stem cell transplant. After a follow-up FDG-PET/CT scan showed no FDG-avid disease, a subsequent study showed FDG uptake in a nonenlarged left axillary lymph node. Questioning elicited a recent immunization history. A follow-up PET/CT scan showed no uptake in this lymph node and no disease recurrence. Without this history, an unnecessary biopsy or treatment may have ensued. Methods to avoid such occurrences are discussed.