Mycobacterium avium complex (MAC) infection is a rarely recognized cause of intrathoracic infection in immunocompetent children. The incidence of this disease is unknown but is likely underestimated among children in whom MAC infection is not usually considered. An increase in the number of cases of MAC infection in adults has been noted since the late 1970s. The number of these cases in children with AIDS has also increased. There are currently no guidelines for the treatment of these children. We describe a previously healthy 14-month-old boy with a mediastinal mass for whom tuberculosis was initially diagnosed; subsequently, biopsy-proven infection with MAC was demonstrated. He received no specific therapy after surgical excision of his intrathoracic mass and has done well since. We reviewed eight additional cases of intrathoracic nontuberculous mycobacteria infection in children that were reported from 1979 to 1994 and found excellent outcomes for seven immunocompetent children who received diverse methods of treatment.