Superior mediastinal syndrome secondary to an anterior mediastinal mass can be seen in acute lymphoblastic leukaemia (ALL) of T-cell lineage. We report a 3-year-old child with B-cell ALL, who presented with the superior mediastinal syndrome. The CT scan chest showed a huge anterior mediastinal mass and the peripheral blood immunophenotyping showed B-cell ALL. High-risk remission induction chemotherapy was given and he achieved remission by the end of induction therapy, both in terms of medullary and extramedullary disease, and is on maintenance chemotherapy now. This is the first reported case of a paediatric B-cell ALL presenting with superior mediastinal syndrome secondary to an anterior mediastinal mass.