Solitary bone cysts are benign, fluid-filled cavities that most often occur in childhood. Several minimally invasive decompression methods have been proposed; however, performing a surgical procedure through the thinned overlying cortex raises the risk of pathological fracture and neurovascular damage, especially in lesions located in the bone diaphysis. We describe a new technique that circumvents these problems: tunneling through the normal cortex and medullary space with a flexible reamer, placing a retrograde medullary nail for cyst decompression.