A subdural hygroma is an accumulation of cerebrospinal fluid in the subdural space that may occur secondary to trauma and surgery, or for iatrogenic reasons, such as a lumbar puncture. Lumbar puncture is a procedure used commonly for intrathecal chemotherapy for patients with B-cell acute lymphocytic leukemia (B-ALL) though subdural hygroma is a very rare complication. We present a case of a fatal, refractory subdural hygroma in a patient with B-ALL.