In this phase I study, teniposide was administered orally for 20 consecutive days to patients with refractory cancers. All patients but one were pretreated. When given for 20 consecutive days, the maximum tolerated dose of teniposide was 100 mg/d. Myelosuppression was the dose-limiting toxicity, and occurred between days 17 and 31. In all patients blood counts had sufficiently recovered by day 35 to begin another 20-day course. Gastrointestinal toxicity persisted in 6 of 15 patients despite antiemetics. Total alopecia was observed in 10 of 13 patients at risk. We recommend teniposide 100 mg/d for 20 consecutive days for evaluation of antitumor efficacy in phase II studies.