Tuberculosis is an infectious disease caused by Mycobacterium tuberculosis complex mycobacteria. Extrapulmonary tuberculosis usually develops more than two years after infection or many years later. Factors favoring onset of the disease are malnutrition, older age, renal failure, diabetes, cancer, immunosuppression and biological treatment, e.g. tumor necrosis factor α (TNF-α) inhibitors. The paper presents a case of a 56-year-old patient with ankylosing spondylitis treated with infliximab, diagnosed with tuberculosis of the spleen. The unusual location and uncharacteristic symptoms created a lot of diagnostic difficulties, particularly as during qualification for biological treatment tests are performed to exclude infection with Mycobacterium tuberculosis. Pharmacological treatment of tuberculosis is typical, but in the case of tuberculosis of the spleen, splenectomy also is a method of treatment. The decision was made to implement pharmacological treatment, which proved to be effective, so the patient avoided surgery.