The article describes the clinical course of a 2-year-old Rumanian girl affected by acute lymphoblastic leukaemia who presented with a severe pneumonia during the initial chemotherapy phase. The main features of the infection caused by the bacterium Legionella pneumophila, a bacterium poorly accounted in paediatric age, are described as well. The paper highlights the importance of bronchoalveolar lavage for the diagnosis of legionellosis and of a prolonged treatment to achieve a substantial clinical and radiological improvement and finally a complete cure.