Pneumonia due to bacteria that do not grow well on routine culture media have been difficult to diagnose. Mycoplasma pneumoniae is an infrequent cause of pneumonia in the adult population. As a cause of atypical pneumonia, along with Chlamydia pneumoniae and Legionella pneumophila, it still makes up about a third of the cases of community acquired pneumonia (CAP). Mycoplasma pneumoniae can cause mild atypical respiratory illness to severe disease requiring hospitalization. It has largely been seen as a disease of children and young adults younger than 30 years. With improved detection systems, more cases in older age groups are expected. This is especially true for immunosuppressed cancer patients who undergo polymerase chain reaction testing of the nasopharynx for respiratory infections. Although detection of Mycoplasma by polymerase chain reaction does not always indicate active infection, immunosuppressed patients who are at higher risk for severe disease and coinfection are more likely to require prompt therapy.