A 25-year-old woman with no underlying disease presented with a large fluid-filled cavitary lesion in the right lung. Mycobacterium celatum was isolated from the cavitary fluid, and treatment with ethambutol, rifampicin, and clarithromycin was initiated. After 4 months of treatment, right lower pulmonary lobectomy was performed. Histological examination confirmed M. celatum infection as well as concurrent congenital cystic adenomatoid malformation (CCAM). M. celatum has been implicated in opportunistic infections. This infection, however, was related to underlying CCAM, which resulted in a large cavitary lesion. CCAM diagnosed in adulthood is rare, and is made more challenging by an infectious complication.