The Authors present a singular case of seroma which developed in residual fibrous capsule after explantation of breast prostheses without replacement. Mammographic and sonographic findings of the seroma make difficult the diagnosis: mammograms revealed a radiopaque, well-circumscribed mass anterior to the pectoralis major muscle in the region of the prior sub-glandular implant, that was initially misinterpreted as breast implant radiographic feature. It is particularly important review and carefully correlate mammographic and sonographic findings with patient's surgical history for a correct diagnosis.