Pneumonias caused by atypical organisms usually have extra-pulmonary features. Chlamydial pneumonia often starts with hoarseness and fever, and respiratory tract symptoms may not appear for days. Mycoplasmal pneumonia may manifest with ear pain and a nonproductive cough. Legionnaires' disease presents with high fevers and central nervous system and gastrointestinal abnormalities. Diagnosis of chlamydial infection is accomplished with serologic testing. Patients are unresponsive to erythromycin treatment and should be started on empirical doxycycline (Doryx, Vibramycin) therapy. The presence of cold agglutinins in the appropriate clinical setting permits a presumptive diagnosis of mycoplasmal infection. Clinical diagnosis of Legionella pneumonia may be made in patients with pneumonia who also have relative bradycardia with elevated serum transaminases or hypophosphatemia with gastrointestinal or central nervous system symptoms. Erythromycin is the mainstay of treatment of legionnaires' disease, but treatment failures have been reported. Doxycycline is less expensive, has a better safety profile, and is better tolerated than erythromycin.