Although their mechanism of action is similar, the preclinical and clinical data of these two drugs have some notable differences. Topotecan and irinotecan have different spectra of antitumor activity in various models of human cancer. Clinical data support that these agents may have different spectra of activity. The differences in antitumor activities may also reflect different mechanisms of resistance. Furthermore, topotecan and irinotecan have different limiting toxicities (myelosuppression and diarrhea, respectively).