Mycoplasma pneumoniae is a pathogenic micro-organism frequently held responsible for acute respiratory infection. The disease is ubiquitous and often proceeds in epidemics among small communities of young people (families, army barracks, universities). Its usual clinical manifestations consist of a stubborn cough symptomatic of tracheo-bronchitis with or without fever, and inflammation of the upper respiratory tract. Cases where chest X-rays show a pulmonary infiltrate are less frequent, but they differ from other lung diseases in that the respiratory signs at physical examination are discreet. The presence of cold agglutinins is not specific, but it contributes to the diagnosis. Cutaneous, neuromeningeal, cardiovascular and osteo-articular manifestations are rare, usually delayed and of lesser importance. Diagnosis rests on positive cultures of tracheo-bronchial or pharyngeal samples and/or on a significant increase in the titers of serum antibodies directed against M. pneumoniae. The disease is usually benign. Antibiotic therapy with macrolides or tetracyclines shortens its duration and reduces the incidence of complications. The latter chiefly concern elderly subjects and patients with COLD for whom M. pneumoniae infection constitutes a major risk of respiratory failure.