An epidemic of Mycoplasma pneumoniae infection has been observed in France since September, 2023. We aimed to describe the characteristics of adults hospitalised for M pneumoniae infection and identify factors associated with severe outcomes of infection. MYCADO is a retrospective observational study including adults hospitalised for 24 h or more in 76 hospitals in France for a M pneumoniae infection between Sept 1, 2023, and Feb 29, 2024. Clinical, laboratory, and imaging data were collected from medical records. We identified factors associated with severe outcomes of infection, defined as a composite of intensive care unit (ICU) admission or in-hospital death, using multivariable logistic regression. 1309 patients with M pneumoniae infection were included: 718 (54·9%) were men and 591 (45·1%) were women; median age was 43 years (IQR 31-63); 288 (22·0%) had chronic respiratory failure; 423 (32·3%) had cardiovascular comorbidities; and 105 (8·0%) had immunosuppression. The most common symptoms were cough (1098 [83·9%]), fever (1023 [78·2%]), dyspnoea (948 [72·4%]), fatigue (550 [42·0%]), expectorations (473 [36·1%]), headache (211 [16·1%]), arthromyalgia (253 [19·3%]), ear, nose, and throat symptoms (202 [15·4%]), diarrhoea (138 [10·5%]), and vomiting (132 [10·1%]). 156 (11·9%) of 1309 patients had extra-respiratory manifestations, including 36 (2·8%) with erythema multiforme, 19 (1·5%) with meningoencephalitis, 44 (3·4%) with autoimmune haemolytic anaemia, and 17 (1·3%) with myocarditis. The median hospital stay was 8 days (IQR 6-11). 424 (32·4%) patients had a severe outcome of infection, including 415 (31·7%) who were admitted to the ICU and 28 (2·1%) who died in hospital. Those more likely to present with severe outcomes of infection were patients with hypertension, obesity, chronic liver failure, extra-respiratory manifestations, pulmonary alveolar consolidation or bilateral involvement on CT scan, as well as elevated inflammatory markers, lymphopenia or neutrophilic polynucleosis, and those who did not versus did receive any antibiotic active against M pneumoniae before admission. This national, observational study highlighted unexpected, atypical radiological presentations, a high proportion of transfers to the ICU, and an association between severity and delayed administration of effective antibiotics. This should remind clinicians that no radiological presentation can rule out M pneumoniae infection, and encourage them to reassess patients early after prescribing a β-lactam, or even to discuss prescribing macrolides as first-line treatment in the context of an epidemic. None. For the French translation of the abstract see Supplementary Materials section.< Réduire