The paper presents the case of a 14-year-old boy with severe microcytic anaemia and lifelong constipation, with imaging showing massive faecal impaction and rectosigmoid dilatation. Poor response to transfusion and iron therapy, together with marked distension, suggested late-presenting Hirschsprung disease. Rectal biopsy confirmed aganglionosis, leading to rectosigmoid anastomosis, which resolved both abdominal symptoms and anaemia. This case underscores the need to consider Hirschsprung disease in adolescents with severe constipation and iron-deficiency anaemia unresponsive to treatment.