Pneumonia is a quite frequent and often fatal complication in patients with acute leukemia. During a 19-month period 52 episodes of pneumonia were found among 68 leukemia patients. Except for fever and chest X-ray abnormalities, presenting signs and symptoms were few and subtle. The responsible infectious agent was isolated in 71% of cases, but only half were found antemortem. Most were Gram-negative bacilli, with a 25% incidence of fungal pneumonia. The overall mortality rate was a disturbingly high 65%, indicating that acute leukemia itself is an important mortality risk factor. Profound granulocytopenia, bacteremia, isolation of either a fungus or Gram-negative bacillus as causative organism, and administration of absorbable or oral nonabsorbable antibiotics prior to development of pneumonia were also significant mortality risk factors. Measures to more effectively treat pneumonia in these patients must include a much more aggressive diagnostic approach, especially in obtaining bronchopulmonary secretions.