The incidence of pneumonia was evaluated in 844 children undergoing initial treatment for acute lymphoblastic leukemia (ALL). A total of 310 episodes occurred in 239 patients followed up for five to 36 months after diagnosis. The peak incidences occurred in the periods 0 to 20 days and 40 to 80 days after the start of antileukemic therapy. Bacterial pneumonias occurred primarily during the first 20 days after diagnosis of ALL. No episode of Pneumocystis carinii pneumonia was noted before 40 days, and the majority of instances occurred 50 to 120 days after diagnosis. In 80% of all episodes, a specific causative organism was not detected. The incidence of P carinii pneumonia was greater in patients receiving intrathecal methotrexate as part of CNS prophylaxis than in those receiving only CNS irradiation. Pneumonia is a frequent event during the therapy of ALL in childhood.