The relation between degree of myelosuppression and episodes of infection was analyzed in 36 patients (92 treatment courses) with small cell lung cancer (SCLC) treated with intensive chemotherapy. The two regimens used were cisplatin (CDDP) + adriamycin (ADR) + cyclophosphamide (CPA) + etoposide (VP-16) + granulocyte-colony stimulating factor (G-CSF) and CDDP + teniposide (VM-26) + G-CSF, and they induced grade 3 or 4 leukopenia in 88% of treatment courses and febrile episodes in 60%. In the febrile courses, the mean nadirs of leukocyte and neutrophils (820 +/- 581/mm3, 101 +/- 267/mm3) were significantly longer (P less than 0.01) and the mean durations of grade 3 and 4 leukopenia and neutropenia significantly longer (P less than 0.001) than those of the non-febrile courses. It was noted, however, that febrile episodes appeared frequently in courses having the nadir of leukocytes below 1,000/mm3 (80%) or the nadir of neutrophils below 100/mm3 (74%). The administration of antibiotics was required for about 7 days to patients with febrile episodes. Sepsis was experienced in five courses, in which the neutrophils were all zero. All the patients, however, could be managed by an administration of antibiotics immediately after a febrile episode appeared, without delaying the subsequent chemotherapy except for one patient, who had had a performance status (PS) of 3 prior to chemotherapy.