We report the case of a 13-year-old girl with Down's syndrome who presented with cough, fever, and a rapidly evolving respiratory failure leading to intensive care unit transfer during coronavirus disease 2019 (COVID-19) outbreak in Belgium. COVID-19-related acute respiratory distress syndrome (ARDS) is rare in teenagers and its diagnosis remains challenging as reverse transcription-polymerase chain reaction (RT-PCR)-testing sensitivity and radiological criteria still have to be defined in that population. We finally concluded to severe Mycoplasma pneumoniae infection. This case report gives the opportunity to discuss the rare occurrence of that disease, and to review the radiological findings of M. pneumoniae-and COVID-19-related pneumonia in teenagers.