Thoracic nontuberculous or atypical mycobacterial infections typically occur in patients who have underlying lung disease or an immunologic abnormality. These infections are usually indolent and the diagnosis is often difficult to establish and, even if confirmed, is of questionable clinical significance. The most common radiologic pattern is fibronodular opacities in the upper lobes similar to those seen with tuberculosis. Less commonly, patients may have scattered nodularity associated with bronchiectasis. If suspected by radiologic and clinical findings, culture should be obtained for diagnosis. This review focuses on nontuberculous mycobacterial disease in the thorax of the immunocompetent host.