Of 148 children with acute lymphoblastic leukemia (ALL) 34 had bilateral interstitial pneumonia (BIP). Their records were reviewed retrospectively to evaluate the incidence of this pneumonia and delineate the various etiologic factors involved. All but 1 were in remission; 82% of the episodes occurred within the first 6 months, the majority of these occurring during the first 3 months of diagnosis of ALL; 16 were receiving methotrexate (MTX), and 14 were receiving combination chemotherapy with vincristine, prednisone and 6-mercaptopurine (6-Mp) and 4 were on no systemic therapy. Thirty patients with 35 episodes recovered within an average period of 18 days, including clearance of radiologic findings; 4 died. Four had open lung biopsies without complications. There was a single case of pneumocystis carinii infection diagnosed postmortem. Laboratory data and histopathologic findings (of 4 biopsies and 3 remaining autopsies) were suggestive of a viral etiology. The incidence of BIP in ALL at Children's Hospital of Michigan is 22.9% with a mortality rate of 10.3%. The incidence of pneumocystis carinii infection appeared to be low. Hypersensitivity to MTX was not substantiated.