67-year-old man with metastatic hepatocellular carcinoma (HCC) treated with nivolumab. A CD8 PET/CT scan acquired 14 days after starting immunotherapy demonstrated increased tracer activity in the primary tumor (arrow; SUVMAX = 22.9) on the coronal PET (A) and fused PET/CT images (B), suggestive of tumor infiltration by CD8+ T cells and a productive anti-tumor immune response; physiologic tracer activity is seen in the spleen, liver, bone marrow, and kidneys. Follow-up imaging demonstrated a partial response to therapy, which has lasted 3+ years, with an associated drop in alpha-fetoprotein from 33.2 ng/mL at baseline to 1.4 ng/mL at 3 years.