Atypical mycobacterial infections may give rise to various clinical difficulties. Case reports of six patients--three adult patients with pulmonary lesions, two children with cervical lymphadentis and one patient in whom the atypical mycobacterium appeared to be present as a commensal--illustrate these difficulties. Determination of the significance of the organism and differentiation of condition from tuberculosis and others requires consideration of the clinical picture, the results of skin testing, histologic features and cultural characteristics. Three patients, including the two children, were treated with a combination of surgery and chemotherapy, with satisfactory results. An elderly patient with chronic bronchitis and a pulmonary infection due to M. kansasii was treated successfully with antiuberculosis agents alone. Chemotherapy is being tried on a fifth patient with cavitary disease due to M. intracellulare, but is seems likely that an operation will also be required.