A phase I study of teniposide administered orally for 5 consecutive days was performed. The first dose was 60 mg/m2/d, with increments of 25 mg/m2/d. Nineteen patients entered the study and received a total of 77 courses with a median of two (range, 1 to 12). Dose-limiting toxicity occurred at 160 mg/m2/d and consisted of myelosuppression, mainly leukocytopenia, and gastrointestinal toxicity. Sufficient recovery of blood counts was seen by day 21 to allow for a repeat course. Two patients, treated with teniposide doses of 110 and 160 mg/m2/d, respectively, were considered toxic deaths. Partial alopecia was frequent at doses above 85 mg/m2/d. Retching and vomiting during administration of the drug were encountered in virtually all patients. Oral teniposide 135 mg/m2/d for 5 consecutive days on a 3-week schedule is recommended for evaluation of antitumor efficacy in phase II studies.