It is well known that biological treatment increases the risk of opportunistic infections. Guidelines recommend tuberculosis screening prior to treatment. This is a case report of a woman who had morbus Crohn and developed peritoneal tuberculosis even though she completed a preventive tuberculosis eradication before initiating treatment with anti-TNF-inhibitor. She appeared with ascites and was examined very thoroughly, and eventually a peritoneal biopsy revealed tuberculosis. Tuberculosis is difficult to diagnose, and eradication is no guarantee that tuberculosis cannot relapse during biological treatment.