Overgrowth syndromes generally present with inherent health concerns and, in some instances, an increased risk of malignant intra-abdominal tumors, such as Wilms tumor or hepatoblastoma. There are various types of hyperplasia, but the crossed type is reported to be the rarest. We present a rare, crossed type of congenital hemihyperplasia. A six-year-old girl was referred to our clinic for leg length discrepancy and was diagnosed with congenital hemihyperplasia of the right lower limb and left upper limb. The leg length discrepancy had begun to affect the patient’s gait and stair climbing. We regularly monitored her leg length and checked for functional and cosmetic problems for 24 months. However, the hyperplasia progressed and caused severe gait impairment. Leg length discrepancy at the last check-up was 30 mm. She underwent femoral lengthening surgery using a monolateral external fixator. At the 14th month follow-up, her gait and stair climbing had greatly improved, and a scanogram revealed that the leg length discrepancy had been successfully treated with a mild varus angulation of the femur. We present a rare, crossed congenital hemihyperplasia. We recommend limb lengthening surgery for treating patients with idiopathic hemihyperplasia.
