We report herein a rare case of delayed infection of a lymphocele following mastectomy with immediate breast reconstruction. A 38-year-old woman presented to our hospital 7 months after undergoing a left-modified radical mastectomy with an immediate breast reconstruction, following the sudden development of a giant mass in the left thoracoabdominal region as well as a high fever and shivering. Ultrasonography and a computed tomographic scan revealed massive fluid retention extending from the left axilla to the lower abdominal region. Puncture drainage was performed three times and the injection of an antibiotic directly into the cyst resulted in resolution of the fluid. This massive retraction of fluid was considered to have resulted from a delayed infection of an axillary lymphocele.