Cystosarcoma phylloides is an unusual breast tumor which presents difficulties in diagnosis with filmmammogram of breast only. Even though histopathologic reports of benign cystosarcoma phylloides, it commonlyrecurrs locally if the lesion is inadequetely excised, and it may metastase hematogeneously. Ultramammogram hasproved useful in the study of breast disease and has permitted the display of some tumor characteritics. Authors analyzed clinical characters of histopathologically confirmed 48 cystosarcoma phylloides and analyzed imagingfinding in 20 cases with imaging studies since 1983 through July, 1988 at Yongdong and Shinchon Severancehospital, College of Medicine, Yonsei University. The results are followed by: 1. Incidence of 48 cystosarcomaphylloides in 2.2% of all admited and operated patients with breast disease, 3.2% of breast tumors and 6.1% ofbenign breast diseases. 2. The age distribution of cystosarcoma phylloides was 12 years old through 55 years old,most commonly in teen age (33.3%) and the mean age was 29.2 years old. 3. Most common symptom duration of papablebresast mass was between 2 month-11.9 months(61.3%) and mean symptom duration was 1.8 years. 4. Incidence ofmalignant cystosarcoma phylloides was 6.2%, recurrence rate was 18.8%, bilaterallity and multiplicity ofcystosarcoma phylloides was 8.3% respectively. 5. Involvement of cystosarcoma phylloides in normal breasts were 47cases and one of cystosarcoma phylloides was involved in right accessory axillary breast. Common location ofcystosarcoma phylloides was central portion of breast, both upper and outer portion, and upper central portionbreast. 6. Mean tumor size of cystosarcoma phylloides was 6.75cm and the commonest size was between 3.1-9.0cm(69.3%), however there was no difference in size between benign and malignant cystosarcoma phylloides. 7. Filmmammographic findings of cystosarcoma phylloides in 16 cases showed DY parenchymal pattern(93.7%), well defined(56.3%), spherical mass(62.5%) with well preserved symmetricity(75%), bulging of skin(62.5%), and halo sign(56.3%), however secondary signs of breast carcinoma were rare findings in cystosarcoma phhylloides. 8.Ultramammogarphic findings were diagnostic in 18 cystosarcoma phylloides; oval(44.4%) or lobulating(38.9%) masswith smooth border(66.7%) intermediate interal echoes(100%), mild posterior enhancement(77.8%), nonuniform andscattered multinodularities surrounded by tree life or cavitary dilalted duetal system which are characteritic findings of cystosarcoma phylloides. 9. 1 case of malignant cystosarcoma phyllodies has been confused with primarybreast cancer in film mammographically, however ultramammographic finding was diagnostic to put exact diagnosis with huge mass, necrotic lesions.