We conducted a multi-institutional phase I clinical study on a 21-consecutive-day oral administration of etoposide in 12 patients with malignant tumor. The initial dose was 25 mg/body/day and then raised to 50 mg/body/day and eventually to 75 mg/body/day. The dose limiting factor was leukopenia and the maximum tolerated dose (MTD) was 75 mg/body/day. Leukopenia was observed in all 4 patients registered at 75 mg/body/day. Two of 4 patients showed WBC count nadirs of less than 2,000/microliters. The two patients had less than 1.5 m2 body surface area. Thrombocytopenia and hemoglobin decrease were mild compared to leukopenia. Other adverse reactions were as follows; anorexia (75.0%), nausea/vomiting (58.3%) alopecia (46.1%). No sign of accumulation after 21-consecutive-day oral administration of etoposide was seen on pharmacokinetic parameters. As for anti-tumor efficacy, a partial response was noted in a patient with lung metastasis of cervical cancer at the 75 mg/body/day dose level. A recommended dose for phase II study was 75 mg/body/day for 21 consecutive days with a one-week interval. However, for a patient with a body surface area less than 1.5 m2 or with a severe prior chemotherapy, 50 mg/body/day would be considered appropriate.