A 68-year-old woman was referred for a nodule in the right lung and hilar and mediastinal lymphadenopathy. Physical examination revealed left cervical lymphadenopathy. 18F-fluorodeoxyglucose positron emission tomography (FDG-PET) images revealed radiotracer uptake in the pulmonary nodule and multiple lymph nodes of the truncus. Biopsies confirmed the diagnosis of synchronous diffuse large B-cell lymphoma and squamous cell lung carcinoma. Because the etiology of hilar and mediastinal lymphadenopathy was unknown, the staging of lung cancer could not be determined. We performed therapy for malignant lymphoma earlier than lung cancer. After therapy, FDG-PET showed that uptake in the lymph nodes due to lymphoma had disappeared, whereas uptake in the pulmonary nodule and right hilar lymph node remained or had increased. Based on these findings, the staging of lung cancer was determined and radical surgery was performed successfully. This rare case of synchronous malignancy illustrates the limitation of initial single testing of FDG-PET.