PURPOSE: To assess the MR findings of siliconomas (silicone granulomas) in patients with interstitial silicone injection mammoplasty. MATERIALS AND METHODS: Women with interstitial silicone injection mammoplasty were referred for this study on the basis of clinical findings of palpable mass. Nine patients with 18 augmentated breasts underwent axial and sagittal MR imaging, and the results were analysed in terms of their size, shape, margin, signal intensity, enhancement pattern, distribution and adjacent parenchymal distortion. We undertook in-vitro MR imaging of silicone, paraffin, fat, and water, and then compared their signal intensities at each sequence. RESULTS: Siliconomas were seen as well-defined low-signal-intensity nodules at T1WI and high-signal-intensity nodules at T2WI. There was no demonstrable contrast enhancement. Where there was breast cancer in which heterogeneous signal intensity was observed at T1 -and T2WI, together with heterogeneous enhancement, siliconomas were well differentiated from the tumor mass. At in-vitro MR imaging of silicone, paraffin, fat and water, paraffin showed a very low signal intensitiy at all pulse sequences but silicone showed low signal intensity at T1-fat-suppressed T1WI and high signal intensity at T2-and water-suppressed T2WI. CONCLUSION: MRI allows clear differentiation of siliconoma from fat and fibroglandular tissue, and can therefore, reveal anatomical details and detect lesions in patients with interstitial silicone injection mammoplasty.