We report a 25-year-old man presenting with high fever, dyspnoea and somnolence. The presence of severe diffuse interstitial pneumonia with extrapulmonary symptoms, such as myositis and subclinical haemolysis, strongly suggested an infection by Mycoplasma pneumoniae. This diagnosis was supported by high titres of cold agglutinins and a positive Coombs test, and directly confirmed by specific IgM serological tests. After initiation of the appropriate antimicrobial treatment mechanical ventilation could be avoided and the patient showed a slow but complete clinical recovery. This diagnosis should be considered in any febrile patient with hypoxaemia and diffuse interstitial pneumonia, and rapid initiation of appropriate antibiotic treatment seems to be crucial for a favourable outcome.