A 6-month-old girl was referred with breast and pubic hair development. Investigations excluded an adrenal or central cause for her precocity. Ovarian ultrasound scans showed bilaterally enlarged ovaries with both solid and cystic changes. A follow-up examination suggested progression of the precocity and in view of the young age of the child, and concerns regarding underlying malignancy, she underwent laparotomy. Histology showed no evidence of neoplasia but there was stromal oedema consistent with a diagnosis of massive ovarian oedema. This entity is poorly recognised in the paediatric literature as a cause of sexual precocity, and has never previously been described in such a young patient. This is an unusual cause of precocity in a young child and its recognition and management are reviewed.