Although fat grafting is expected to become a common and widely used procedure in breast reconstruction in the future, there is little known about its effects at the follow-up after initial breast cancer treatment. We performed ultrasound after fat grafting and assessed the findings. On ultrasound of 126 breasts in 116 cases of breast reconstruction with fat grafting performed at our facility, tumorous lesions were found in 51 breasts and no such lesions were found in 75 breasts. In total, 151 tumorous lesions were identified. These comprised 145 cystic lesions, three mixed lesions, and three solid lesions. One of the three solid lesions was pathologically examined and was found to be benign. When fat grafting is performed for breast reconstruction, it is known to cause fat necrosis. Depending on the range of necrosis and shape, changes, such as cyst formation or scar healing, may occur and be observed on ultrasound. Although most cases are benign, a limited number of cases exhibit findings of malignancy. Therefore, the above findings need to be shared between plastic surgery and breast surgery departments.