A 6.5-month-old girl presented to a pediatrician for a second opinion regarding worsening macrocephaly and developmental regression. She then underwent neurosurgical evaluation. Rapid-sequence magnetic resonance imaging was significant for benign enlargement of the subarachnoid space of infancy. A complete blood count was significant for 33% blasts in the peripheral blood. Flow cytometry of the peripheral blood established a diagnosis of T-cell Acute Lymphoblastic Leukemia (ALL). T-cell ALL has been rarely reported and our patient’s presentation with macrocephaly is particularly unique.