Infections are the most common causes of fever of unknown origin (FUO). Both human immunodeficiency virus (HIV) itself and its concomitant opportunistic infections cause FUO. Clinically, the symptoms and findings are often elusive and difficulties are encountered in diagnosis and treatment. A 40-year-old male was admitted to our outpatient clinic with fever, shortness of breath, dry cough increasing at night and weight loss. The patient was hospitalized with FUO and was diagnosed as HIV and tuberculosis coinfection. The HIV RNA level of the patient was 3 892 819 copy/mL and the CD4 count was 97/mm(3). Antitetroviral therapy was postponed for 8 weeks in the light of current guidelines and antituberculosis treatment was started. As this patient showed that HIV status of patients with tuberculosis should be established. Furthermore, both the drug-drug interactions and development of immune reconstitution inflammatory syndrome should be considered in the treatment of HIV-infected patients with tuberculosis coinfection.