Eleven cases of poorly characterized nonneoplastic endocardially based intracavitary cardiac masses have been seen at the Mayo Clinic between 1965 and 1994. They occurred in 7 women and 4 men from age 16 to 75 years (mean, 52 years). The patients presented with a wide variety of symptoms and underlying diseases. The lesions were diagnosed as a primary cardiac neoplasm in 6 of 8 patients having echocardiography. Surgical excision was the treatment of choice in 10 patients; the 11th patient died of noncardiac causes 30 days after the mass was discovered. Grossly, the lesions were firm, yellow-white, and partially calcified, and arose in any of the four chambers. Microscopically, all lesions were characterized by nodular calcium in a background of degenerating blood elements and chronic inflammation. All patients had a benign course relative to their cardiac lesion. Repeat echocardiogram in two patients showed residual calcium in the region of the initial tumor. Although these lesions may represent calcified thrombi, the clinical presentation did not suggest thrombosis as the most likely explanation for their occurrence. Based on the combined clinical presentation and microscopic appearance, the authors propose the descriptive name calcified amorphous tumors (cardiac CAT) to describe this group to nonneoplastic cardiac masses.