Clinical manifestations, diagnostic studies and management of chlamydial pneumonia were reviewed in 115 infants and compared to those from 21 infants with interstitial pertussoid eosinophilic pneumonia. The identity of these two forms of subacute afebrile pneumonia is suggested. Chlamydial pneumonia is natally acquired, essentially occurs during the second and third month of life, and its frequency in the United States is surprisingly high. Gradual onset of respiratory tract symptoms, lack of possible infection from symptomatic persons, afebrile course and staccato cough represent the typical anamnesis. Presence of conjunctivitis and serous otitis is common. Distinctive diagnostic studies include slight eosinophilia (greater than or equal to 300 eosinophils/mm3), elevated serum immunoglobulins M and G, depressed PO2 and normal PCO2 in arterial blood under room air, and both interstitial infiltrates and hyperexpansion of the lungs on chest roentgenogram. Chlamydial cultures of nasopharyngeal secretions or tracheal aspirates, and determination of antichlamydial antibody titers in serum are indicated in establishing the etiology. Adequate management consists of antichlamydial chemotherapy and general supportive care, including chest physiotherapy and oxygen and parenteral fluids when needed.