Irinotecan is an analogue of camptothecin, a topoisomerase I inhibitor, that has an important role in the management of advanced colorectal cancer. It is approved as first-line therapy in combination with 5-fluorouracil and leucovorin or as monotherapy in the second-line setting. Its clinical use has been associated with variability in terms of pharmacokinetic behavior and toxicities, especially myelosuppression and diarrhea. Irinotecan is metabolized to the active compound SN-38; it is now known that the metabolism of irinotecan and the inactivation of SN-38 by glucuronidation are mediated by genetic differences, which contributes to the variability. In this article, a case is presented that illustrates this, and there is a brief discussion of the clinical pharmacology of irinotecan and some of the genetic variations that affect its cellular metabolism.