Abstract: Mycobacterium celatum has been shown to cause fatal disease in both immunocompetent and immunocompromised patients. The similarity in symptoms and radiological features between M celatum and M tuberculosis makes it difficult to differentiate between the 2 clinically or based on early laboratory testing. Clinical symptoms include cough, malaise, and weight loss associated with cavitary lesions and pulmonary infiltrates, whereas the initial laboratory testing includes positive acid-fast stain. Identification using nucleic acid amplification testing (NAAT) in the diagnostic process is sensitive; however, cross-reactivity between M celatum and M tuberculosis NAAT leading to diagnostic errors and inappropriate treatment has been reported. We report a case of pulmonary disease caused by M celatum misdiagnosed as M tuberculosis based on NAAT.