A 36-year-old man underwent FDG PET/CT to evaluate a lung mass in the right middle lobe. The mass on the early PET images that were acquired 50 minutes post-FDG injection had only minimally increased FDG activity with uptake ratio between the mass and mediastinal blood pool of 0.93. However, on the delayed images acquired 230 minutes post-FDG administration, this ratio increased to 3.0, which enabled better notion of the lesion. Pathological examination demonstrated a primary pulmonary mucinous adenocarcinoma.