A case of sarcoidosis or sarcoid reaction with a rare manifestation of unilateral lymphadenopathy compressing the trunk of the right pulmonary artery is presented. A 71-year-old woman was admitted for evaluation of a left hilar mass. Chest CT scans showed a mass invading the right pulmonary artery. A frozen section obtained following open lung biopsy showed lymph node tissue largely replaced by noncaseous granulomas indicating sarcoidosis or sarcoid reaction. Old uveitis was compatible with sarcoidosis, and no malignancy was evident. These findings suggested sarcoidosis, however, other evidence of sarcoidosis was not obtained.