Cardiac rhabdomyomas are rare tumors of infancy. In a series of 36 patients 78 percent were under 1 year of age, and only one patient was over age 15 years. Ninety percent of the rhabdomyomas were multiple and occurred with nearly equal frequency in the right and left ventricles. Although reportedly infrequent in the atria, rhabdomyomas involved either one or both atria in 30 percent of patients. In 50 percent of patients at least one of the tumor masses was intracavitary and obstructed 50 percent or more of one of the cardiac chambers or valve orifices. Symptoms referable to obstruction of intracardiac blood flow were present in nine patients, none of whom had tuberous sclerosis, and all of whom would appear to have been good surgical candidates. Histologically the rhabdomyomas were composed of classic "spider cells". Electron microscopic studies revealed scattered bundles of myofibrils ringing these cells and radiating toward the center; glycogen was present both free in the cytoplasm and within mitochondria. Distinct intercellular junctions resembling intercalated discs with well defined desmosomes and nexuses were present. Many of the cells contained leptofibrils, arranged either peripherally or in spiraled clusters in the center of the cell. Rhabdomyomas derive from cardiac muscle cells and appear to represent hamartomas rather than true tumors.