The UGT1A1 *28 allele results in slower irinotecan glucuronidation, and thus greater exposure to its active metabolite SN-38. At the standard irinotecan dose used in FOLFIRI (180 mg/m2; established prior to our understanding of the importance of genotype in the rate of this drug's metabolism), there is a small increased risk of neutropenia in *28 homozygotes. However, the risk of clinically important consequences of neutropenia, such as febrile neutropenia and infection, are not significantly increased. Patients with other genotypes have a quite low risk of adverse effects suggesting patients with these low risk genotypes may tolerate higher doses of irinotecan in FOLFIRI. This finding was demonstrated in a phase I study in which *1/*28 and *1/*1 genotypes were able to tolerate escalating doses of irinotecan up to 260 mg/m2 and 310 mg/m2, respectively.