The latissimus dorsi musculocutaneous flap (LDMCF) is widely used for breast reconstruction. However, it has the disadvantage of frequent seroma formation at the donor site, and late seroma has also been reported. The authors report histological findings after the surgical treatment of a late, repeatedly recurrent seroma at 10 years after breast reconstruction with LDMCF. In 2008, a 66-year-old female patient underwent immediate breast reconstruction with LDMCF. In 2015, a late seroma was found at the donor site. After aspiration and drainage, the seroma recurred again in 2018. Total surgical excision of the seroma was performed and bloody-appearing fluid was identified in the capsule. The excised tissue was biopsied. Histological examination revealed no evidence of blood in the fluid, and multinucleated giant cells with amorphous eosinophilic proteinaceous material were identified. The cyst was suggestive of chronic granulomatous inflammation. There was no recurrence at 8 months postoperatively. The patient described herein underwent surgical treatment of late seroma that recurred after immediate breast reconstruction with LDMCF, and histological findings were identified. These results may be helpful for other future studies regarding late seroma after breast reconstruction with LDMCF.