A 61-year-old woman who was negative for type 1 human immunodeficiency virus developed vertebral osteomyelitis and skin lesions due to sepsis by Staphylococcus aureus. Microscopic examination of the skin showed alcohol-resistant acid-fast bacilli. A polymerase chain reaction (PCR) assay for Mycobacterium tuberculosis was positive for skin and spinal samples, although the cultures were negative. The diagnosis of M. tuberculosis infection is difficult, particularly when the disease is extrapulmonary. Rapid diagnostic tests that use PCR identify the DNA of the bacillus with greater sensitivity than microscopic examination and can give results within 24 hours of receipt of a sample. We analyze the utility of PCR for diagnosing extrapulmonary tuberculosis.