Lung cancer associated with Sweet's syndrome is extremely rare. There are only seven reports of such cases. As far as could be determined from a comprehensive search, there is no reported operative case of lung cancer with this syndrome in the world literature. A 75-year-old Japanese man was diagnosed as having Sweet's syndrome. A chest computed tomography (CT) scan to screen for malignant lesions associated with this syndrome revealed an abnormal shadow in the lung. Although [(18)F]2-fluoro-2-deoxy-D: -glucose positron emission tomography showed no abnormal uptake, lung cancer was most strongly suspected by chest CT. His erythema improved rapidly with steroid therapy and he underwent a segmentectomy (S(6)) of the right lower lobe. A pathological examination revealed lung adenocarcinoma (pT1N0M0: Stage Ia). The patient was discharged from the hospital without any worsening of Sweet's syndrome. We herein report a first operative case of an early stage lung adenocarcinoma with this syndrome.