The Authors report the case of a 9 year-old girl suffering from acute abdominal pain, combined with mild anaemia (Hb 10.9 g/dL), leukocytosis (24.3 x 103 cells/dL), and a large palpable mass in the upper left quadrant. The child underwent an appendectomy 20 days before the admission to our Department. The operation performed in urgency, as well as the removal of a bulky mass situated in the left flank and the right ovary whence it arose, made it clear that abdominal signs and symptoms were caused by the twisting and rupturing of a neoformation, that would hence cause an impressive hemoperitoneum. The histopathologic examination showed a three-germ layer mature mixed teratoma. Clinical, radiologic and biochemical test (α−FP, β−hcG) performed in a postoperative 2 months follow-up revealed no residual disease.