The authors present a patient with simultaneous follicular thyroid and small-cell lung cancers, both of which showed Tc-99m MIBI uptake. CT scans showed two masses: one involving the right lower neck including the right supraclavicular area and the right superior mediastinum, and the other involving the peripheral portion of the right upper lobe of the lung. I-131 imaging showed increased uptake in the right neck mass only. Tc-99m MIBI imaging, which was performed for evaluation of chest pain, showed intense uptake in the neck mass (tumor to heart ratios in planar and tomographic images were 0.92 and 0.96, respectively), and less uptake in the lung mass (tumor to heart ratios in planar and tomographic images were 0.53 and 0.40, respectively). Biopsy of the right supraclavicular mass revealed a follicular carcinoma, and a bronchoscopic biopsy of the right upper lobe mass revealed a small cell carcinoma.