Macromastia and in particular unilateral macromastia is a rare clinical entity. It relates to massive enlargement of the breast in non-obese women. This case report describes an initially mild unilateral asymmetry occurring nine months postpartum in a 33-year-old female. However, following her second pregnancy within 12 months, her left breast became severely enlarged. This did not improve on delivery. No discrete lesion was seen on imaging and no significant abnormalities were seen in her blood chemistry. Surgical treatment was a mammoplasty and 580 g of mammary tissue was removed. Grossly, there was spongiform subcutaneous tissue with diffuse extension. On histology, this consisted of a highly complex and diffuse pattern of infiltration and of cavernous empty channels lined by a delicate attenuated endothelium which was CD34 and D2-40 positive. The appearances were consistent with lymphangiomatosis, more commonly encountered in the limbs, heretofore. Lymphangiomatosis has not been previously described in breast tissue and only a single case report exists for such a lesion in axillary tissue. Treatment of such lesions in the periphery by surgical excision is very difficult and excision without being radical can be impossible. Follow up of our patient, shows no evidence of recurrence in this patient. Our case report describes the clinicopathological features, differential diagnosis to be considered and treatment, in addition to reviewing the relevant literature.