Arachnoid cyst is a common congenital anomaly in the pediatric population. The cysts are often asymptomatic, but they can cause headache and other symptoms. Occasionally a cyst may rupture after head trauma producing a subdural hematoma. The authors present the case of an 11-year-old boy who presented after a week of progressive and severe back pain radiating to the back of his thighs. Imaging revealed a spinal subdural blood collection at the L4-S1 level. This finding prompted further cephalad imaging of the spine and the brain, which revealed a sylvian fissure arachnoid cyst with intracystic hemorrhage and frontoparietal subdural hematoma. The child did not have headache at this time, although he had experienced chronic headaches since the age of 4 years. He was treated with a course of oral steroids, which immediately relieved his back and leg pain. Subsequent imaging showed resolution of the cranial and spinal subdural blood collections and diminished size of the arachnoid cyst. No surgical treatment was necessary.