A 10-year-old boy who had been treated for acute lymphoblastic leukemia presented with persistent numbness of the left big toe and progressive pain of the ipsilateral lower leg. He had received allogeneic bone marrow transplantation 3 months after a testicular relapse. He was in hematologic remission at admission but as progressive swelling of his left leg continued, bone marrow relapse developed. A muscle biopsy revealed leukemic infiltrates in the surrounding muscles of the left sciatic nerve, and swelling of the nerve was found on a magnetic resonance imaging scan. His symptoms/signs subsided soon after reinduction chemotherapy. Unfortunately, he didn't survive because of a fungal sepsis that developed during the neutropenic state. This case represents a rare neurologic complication of what is currently an uncommon presentation for relapse of acute lymphoblastic leukemia, with acute sciatica and without coexisting epidural or leptomeningeal leukemia.