Cystosarcoma phylloides is an unusual breast tumor which presents difficulties in diagnosis with film mam-mogram of breast only. Even though histopathologic reports of benign cystosarcoma phylloides it commonly recurrs locally if the lesion is inadequetely excised and it may metastase hematogeneously. Ultramammogram has proved 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 imaging findings in 20 cases with imaging studies since 1983 through July 1988 at Yongdong and Shinchon Severanc hospital College of Medicine Yonsei University. The results are followed by : 1. Incidence of 48 cystosarcoma phylloides is 2.2% of all admited and operated patients with breast disease 3.2% of breast tumors and 6.1% of benign 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 papable breast mass was between 2month-11.9months(61.3%) and mean symptom duration was 1.8years. 4. Incidence of malignant cystosarcoma phylloides was 6.2% recurrence rate was 18.8% bilaterality and multiplicity of cystosarcoma phylloides was 8.3% respectively 5. Involvement of cystosarcoma phylloides in normal breasts were 47 cases and one of cystosarcoma phylloides was involved in right accessory axillary breast, Common location of cysosarcoma phylloides was central portion of breast both upperand outer portion and upper central portion breast. 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. Film mammographic 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 phylloides. 8. Ultramammographic findings were diagnostic in 18 cystosarcoma phlloides; oval (44.4%) or lobulating (38.9%) mass with smooth border(66.7%) intermed diate internal echoes (100%) mild posterior enhacement (77.8%) nonuniform and scattered multinodularities surrounded by tree like or cavitary dilated duetal system which are characteritic findings of cystosarcoma phylloides. 9. 1 case of malignant cystosarcoma phyllodies has been confused with primary breast cancer in film mam-mographically however ultramammographic finding was diagnostic to put exact diagnosis with huge mass necrotic lesions.