The somatostatin analog, octreotide, is effective in treating diarrhea associated with cancer chemotherapy. This study was undertaken to determine whether octreotide could be used as prophylaxis against chemotherapy-induced diarrhea and, thereby, permit increased dose intensity. Adult cancer patients were treated with a standard regimen of intravenous 5-fluorouracil (5-FU) (600 mg/m2) plus leucovorin (LV) (500 mg/m2) weekly x 6 weeks. In addition, 150 microg of octreotide was administered subcutaneously twice daily, beginning on the first day of chemotherapy and continuing for 43 days. Escalation of 5-FU was planned for successive cohorts based upon toxicity. Eleven patients were treated at the initial 5-FU dose level. In 10 evaluable patients, dose-limiting toxicities were diarrhea (two patients), fatigue (one patient), and hyperbilirubinemia (one patient). Diarrhea was experienced by six of 10 patients, and only three patients were able to receive six weekly chemotherapy treatments without dose reduction or delay. At a dose of 150 microg twice daily, octreotide did not prevent diarrhea associated with 5-FU plus LV, and 5-FU dose escalation was not possible. While octreotide is successful in the treatment of 5-FU-induced diarrhea, we were unable to demonstrate a role in toxicity prophylaxis.