Mediastinal tuberculous lymphadenitis is a relatively rare disorder and it is sometimes hard to identify the acid-fast bacilli involved or its specific pathological findings. We employed thoracoscopy to obtain diagnostic material from a 25-year-old woman with a newly-appearing mediastinal mass despite antituberculous therapy for cervical tuberculous lymphadenitis. Thoracoscopy also proved effective in the curative surgical procedure, i.e. resection of the abscess wall as far as possible, debridement of the remaining wall, irrigation, and instillation of streptomycin into the abscess and thoracic cavity. Postoperatively the patient has followed a favorable course for one year.