Ten patients with sepsis and pneumonia complicated by leukemia or lung cancer were treated with cefoxitin (CFX) at daily dose of 6 g. The following results were obtained. 1. Clinical effects of CFX were good in 5 patients, fair in 2 and poor in 3 with effective rate of 50%. 2. Out of 8 patients with sepsis, 5 showed good response to CFX and effective rate was 62.5%. 3. Bacteriological outcomes were eradicated in 1, unchanged in 1, replaced in 2 and unknown in 6 cases. 4. Diarrhea was observed in 1 patient but this was not considered related to CFX therapy. 5. No abnormal laboratory finding due to CFX was observed. 6. It should be considered that 6 g or more of CFX is given in case of severe infections, such as sepsis or pneumonia complicated by serious underlying diseases.