A previously healthy 12-year-old boy presented with acute onset of proptosis of his left eye. CT scan demonstrated a mass involving the left orbit, left maxillary sinus, and left ethmoid sinus with extension through the cribriform plate into the anterior cranial fossa. Incisional biopsy of the mass revealed a precursor B-cell lymphoblastic lymphoma. Precursor B-cell lymphoblastic lymphoma is a rare type of non-Hodgkin's lymphoma seen exclusively in children and young adults. This is the first reported case of precursor B-cell lymphoblastic lymphoma presenting in the orbit. Treatment is primarily by systemic chemotherapy and is potentially curative. The principal role of the ophthalmologist is in diagnosis and monitoring of such patients. The clinical features and multidisciplinary diagnosis and management of childhood non-Hodgkin's lymphomas are reviewed.