Between Jan 1 and Oct 31, 1975, a cluster of ten cases of pneumocystis pneumonia occurred in children with acute lymphocytic leukemia (ALL) at the James Whitcomb Riley Hospital for Children in Indianapolis. The risk of infection appeared to be related to the intensity of chemotherapy. Furthermore, illness developed in nine of the ten patients between 30 and 100 days after initiation of therapy, suggesting a period of heightened susceptibility to infection. An indirect immunofluorescent test was used to detect antipneumocystis antibodies in serum samples collected from patients with pneumocystis pneumonia and their contacts. Members of the Riley Hospital staff who had close contact with infected children had a higher prevalence of elevated antibody titers (7/12) than other staff members (2/22; P = .004) or parents of infected patients (0/8; P = .01). This suggests that transmission of pneumocystis may occur within the hospital environment.