PURPOSE: To evaluate the sonographic, mammographic and MRI features of pregnancy-associated breast cancer with the major focus on the sonographic benefit in a diagnosis of pregnancy associated breast cancer. MATERIALS AND METHODS: From 1998 to 2002, sonography was performed on a total 7 patients (age 23 to 38 years), who were pathologically diagnosed with breast cancer during pregnancy. Six of those patients underwent mammography. Five patients underwent a breast MRI, preoperatively. The radiological findings were evaluated retrospectively. Six patients underwent surgery and 1 patient underwent a core biopsy and chemotherapy. The histological, nuclear grading and pathological staging were evaluated. RESULTS: The sonographic findings showed a mass with irregular shapes (n=6), irregular margins (n=6), a non-parallel orientation (n=5), complex echo patterns (n=5). Associated findings could be observed in 3 patients, including Cooper's ligament thickening (n=2), edema (n=2), skin thickening (n=1) and axillary lymphadenopathy (n=3). The sonographic findings were positive and showed masses in 6 patients. All the patients had a dense breast in mammography. The mammographic findings included masses (n=4), masses with microcalcifications (n=2), masses with axillary lymphadenopathy (n=3), calcifications alone (n=1), an asymmetric density alone (n=1), extremely dense breasts with negative findings (n=2). A breast MRI showed an irregular shaped mass (n=4) with a rim-like enhancement (n=3), linear ductal enhancement without a mass (n= 1), and the time intensity cure revealed the typical pattern and level of enhancement in the carcinoma. CONCLUSION: Sonography is a valuable tool for diagnosing pregnancy-associated breast cancer. However, mammography should be performed if there is a suspicious lesion on sonography in order to confirm the pregnancy-associated breast cancer. Mammography has a lower sensitivity during pregnancy due to the physiologic changes in the breasts. However, calcifications and associated findings are helpful in confirming pregnancy-associated breast cancer. physicians should not consider a rapidly growing lump in the breast during pregnancy to be benign and should perform radiological examinations and imaging-guided biopsies.