A 66-year-old woman diagnosed as having primary lung cancer with an enlargement of the mediastinal, subcarinal and bilateral hilar lymph nodes underwent a thoracotomy after preoperative chemotherapy. The histological examination showed a moderately differentiated papillary adenocarcinoma in the left upper lobe without any metastases to the regional lymph nodes; however, non-caseous epithelioid granulomas without necrosis were observed in the nodes. These changes were considered to be sarcoid reactions, since no other physiological and laboratory findings compatible with systemic sarcoidosis were observed. The histological findings suggested the possibility that the regional lymph nodes were not involved with the tumor cells at the time of lung cancer diagnosis. Despite the rarity of this condition, sarcoid reactions in the regional lymph nodes must be kept in mind when an enlargement of bilateral hilar lymph nodes is detected in patients with lung cancer, and a good prognosis may be expected in cases with sarcoid reactions in the regional lymph nodes.