Background: To evaluate the imaging characteristics of primary lymphoma of the breast. Materials and methods: We reviewed the clinical and imaging findings of seven women with primary lymphoma of the breast between 1991 and 1999. Six of the patients had presented with rapidly growing palpable breast masses. Mammography was performed before excisional biopsy in all patients. Ultrasonography (US) and computed tomography (CT) scans were each performed in four patients. Histopathologic study of the biopsy specimens confirmed the presence of malignant lymphoma in all of the patients. Results: Mammographic findings ranged from solitary, uncalcified discrete round masses (3 cases), to a multiple uncalcified mass (1 case) and diffuse increase in parenchymal density (2 cases). No definite abnormality was seen on the mammograph in one patient. Secondary signs, such as microcalcification or surrounding architectural distortion, were not predominantly seen. In the four cases that were hypoechoic to isoechoic by US, two of the masses were multiple nodular and two were diffuse-mass type. CT scanning showed either ill-defined (3 cases) or well-defined masses (1 case). Three cases showed good enhancement. Histopathologic study of the biopsy specimens confirmed diffuse large-cell lymphoma in five cases, and mixed small- and large-cell lymphoma, or mixed diffuse large-cell and high-grade mucosa-associated lymphoid tissue lymphoma in one case each. Conclusion: Although no findings are pathognomonic by mammography, US or CT scan, the presence of a single or multiple unilateral, relatively large, rapidly-growing palpable mass that exhibits diverse radiologic findings with a lack of accompanying microcalcifications and secondary signs suggests the differential diagnosis of primary lymphoma of the breast.