Reactivation of tuberculosis is a severe side effect of
anti-TNF treatment. Especially extrapulmonary forms of
tuberculosis may occur, which are difficult to diagnose.
The diagnosis may be obtained by a thorough search for
<i>Mycobacterium tuberculosis</i>. We describe two patients who developed tuberculous peritonitis after infliximab therapy that was prescribed for treatment of rheumatoid arthritis. These cases illustrate that tuberculous peritonitis has a nonspecific clinical manifestation and that Mycobacteria can be difficult to find in ascites fluid. For this reason, tuberculostatic therapy has to be started in case of clinical suspicion. Before starting infliximab therapy, the patient must be thoroughly screened for the presence of (latent) tuberculosis.