An asymptomatic congenital cystic adenomatoid malformation of lung discovered at age 18 months, and unchanging for 14 months prior to its removal, was found to harbor a small embryonal rhabdomyosarcoma following excision. With lobectomy and chemotherapy, the tumor-free interval has now exceeded three years. The case represents a well-documented occurrence of malignant tumor arising within a congenital malformation, an association that is unlikely to be the result of chance.