A 62-year-old woman with liver cirrhosis developed ascites. She had been previously treated with a combination of interferon and ribavirin therapy. The ascites was bloody and of exudative nature. Radiological examinations showed supraclavicular, axillar, and mediastinal lymphadenopathy. Biopsy of the axillar lymph node was performed because of suspected malignancy, and the results showed that the lymph node had granulomatous inflammation with caseous necrosis and Langhans giant cells, suggestive of mycobacterial infection. Furthermore, a DNA sequence specific to Mycobacterium tuberculosis was recovered from the same lesion, leading to a diagnosis of tuberculous lymphadenitis. The ascites and the lymphadenopathy subsided with anti-tuberculosis chemotherapy. Although bacilli were not detected in the ascites, a high level of adenosine deaminase in the ascites, the coexistence of tuberculous lymphadenitis, and the response to anti-tuberculosis agents supported the diagnosis of tuberculous peritonitis. Although tuberculous peritonitis is often difficult to diagnose, lymph node biopsy was useful to establish the diagnosis in the present case.