We report a rare case of gravid macromastia (GM) treated after delivery. A 23-year-old woman with left breast enlargement was referred to our hospital. Laboratory investigations revealed an elevated serum CA19-9 level of 200.3 U/ml. Gravid macromastia was diagnosed by clinical examination (US, MRI) and 11G mammotome biopsy. Tumorectomy of the left breast and mammoplasty were performed. The mass had proliferated to the exclusion of normal gland. Histological examination revealed abundant proliferation of stromal loose connective tissue and interposed fat cells surrounding normal and lactating lobules. The epithelial cells were positive for CA19-9 on immunohistochemical staining. Serum CA19-9 decreased to normal levels 3 months after operation. GM is exceedingly rare and occurs in less than 0.001-0.003% of all pregnant women. Once established, the condition is likely to continue to successive pregnancies. This benign tumor is sometimes difficult to distinguish from malignant tumors since it shows few histological characteristics and grows rapidly. Attention is necessary to diagnose tumors of the breast during pregnancy.