Five extracardiac rhabdomyomas were studied by light and electron microscopy: two of the adult type, two of the fetal type, and one of the female genital tract type. The three types can be distinguished histologically and ultrastructurally. Adult type rhabdomyomas are encapsulated and reveal histologically closely packed, large tumor cells with granular or vacuolated cytoplasm and only scant stroma. Electron microscopically, myofibrils are haphazardly arranged with rodlike Z-band material. Sometimes a sarcomeric arrangement of myofibrils is seen. The tumor cells are packed with mitochondria bearing lamellar inclusions. Fetal type rhabdomyomas contain a mixture of muscle cells in different stages of differentiations; the amount of stroma is increased. The myofilaments are usually disorganized and in only a few cells is rodlike Z-band material found. The cytoplasm reveals few mitochondria. In the rhabdomyoma of the female genital tract, the tumor cells are large and resemble more mature muscle fibers; the stroma is abundant and the myofilaments are usually arranged in an orderly fashion. Generally, the prognosis for rhabdomyomas is good. Metastases are not known. Two of the five rhabdomyomas studied recurred. One rhabdomyoma was found at autopsy. Embryonal rhabdomyosarcoma must be considered in differential diagnosis particularly for fetal type rhabdomyoma.