The clinical and radiological characteristics of 217 consecutive episodes of gram-negative bacillary pneumonia occurring in 189 adult cancer patients between November 1968 and December 1974 were analyzed. The majority of patients had acute leukemia (54%). Fever larger than or equal to 101 degrees F was the single most common symptom and sign of the presence of infection (90%). Next in frequency were crepitant rales (65%), cough (41%), dyspnea (19%) and chest pain (18%). Radiographic evidence of pneumonia was found in 83% of cases and it consisted mainly of alveolar infiltrates involving both lung fields and predominantly the bases. Up to one-third of the patients had normal chestx-ray examinations at the onset of infection, though they subsequently became abnormal in 42% of them. The majority of patients (81%) whose initial chest x-rays did not reveal the presence of pneumonia were neutropenic (less than 1000 circumlating neutrophils/mm3). Klebsiella sp. and Pseudomonas sp. were the most common infecting organisms. The overall cure rate was 61%; 70% for Klebsiella sp. infections and 64% for Pseudomonas sp. infections. Pulmonary abscesses occurred in 14% of the cases. Cures were related to the antibiotic sensitivity of the infecting organisms and to the number of circulating neutrophils during the period of infection. Best results were obtained with the administration of gentamicin, the newer aminoglycoside antibiotic sisomicin, tobramycin and amikacin, or the combination of gentamicin with carbenicillin or with cephalosporins. Early and vigorous therapy of gram-negative bacillary pneumonia with appropriate antibiotics has improved the prognosis of this infection at our institution.