Primary carcinoid tumor of the thymus is a rare, slow-growing neoplasm occurring most commonly in middle-aged men. The usual clinical manifestations are those of a mediastinal mass, although in asymptomatic patients, the diagnosis may be suggested by a routine chest roentgenogram. Excision is the most common treatment. Because the disease may recur up to ten years after surgery, follow-up is important. The case reported here is of a 49-year-old man who was hospitalized for evaluation of a mediastinal mass first detected six years previously. When laboratory and other tests proved nondiagnostic, thoracotomy was done and pathologic examination of the excised mass revealed carcinoid tumor of the thymus.