We describe a 69-year-old male patient who was referred for the investigation of long-lasting fever, anemia and neutropenia. Hairy cell leukemia was diagnosed and treated successfully. However, fever persisted despite thorough investigation and use of broad-spectrum antibiotics. Four months after the initial diagnosis, the patient underwent explorative laparotomy and splenectomy. Spleen biopsy revealed multiple necrotizing mycobacterial granulomata while the patient's fever disappeared permanently. Isolated splenic mycobacterial disease is very rare. This case report emphasizes that investigation of chronic fever in hairy cell leukemia requires a high level of clinical suspicion. Early diagnostic procedures for evidence of atypical mycobacterial infection should be considered. When everything else fails, surgery can be helpful in selected cases.