During a 10-year period, 621 episodes of Escherichia coli bacteremia occurred in 575 cancer patients. The infection was most common in patients with acute leukemia and genitourinary and gastrointestinal malignancies. Most of the patients acquired their infection while in the hospital, and 38 percent had received antibiotics during the preceding 10 days. Fever occurred in 96 percent of patients, and afebrile patients had an especially poor prognosis. Only 4.5 percent of the patients had disseminated intravascular coagulation, although hemorrhage contributed to the death of 15 percent of the patients. The overall response rate was 66 percent, but it increased from 48 percent in 1972 to 76 percent in 1981. Patients without pulmonary infection had a response rate of 78 percent, whereas patients with pulmonary infection had a response rate of only 41 percent. Patients who had positive blood culture results while receiving appropriate antibiotic therapy had a poor prognosis. There was no correlation between the patients' initial neutrophil counts and response, but patients whose neutrophil count increased during therapy had a response rate of 75 percent, compared with a 47 percent response rate for patients whose neutrophil count decreased. The response rate was 71 percent for patients who received appropriate antibiotics, 38 percent for patients who received inappropriate antibiotics, and 8 percent for patients who received no antibiotics. A single appropriate antibiotic was as effective as a combination.