Many studies have demonstrated the impact of UGT1A1 on toxicity of irinotecan. In particular, patients bear-ing UGT1A1*28 (TA 7/7) have a higher risk of severe neutropenia and diarrhea. Based on this, prescribers of irinotecan are advised that patients with UGT1A1*28 (TA 7/7) should start with a reduced dose of irinotecan, although a particular dose is not specified. Research in Asian countries has shown a lower incidence of UG-T1A1*28 (TA 7/7), while UGT1A1*6 (A/A) is more often found and is associated with severe irinotecan-related neutropenia. We report here a case of a metastatic colorectal cancer patient who is heterozygous for the UGT1A1*28 polymorphism (TA 6/7) as well as the UG-T1A1*6 polymorphism (G/A). The patient was treated with FOLFIRI for 9 cycles and underwent two irinote-can dose reductions according to pharmacokinetic data regarding exposure to the active metabolite, SN-38. Simultaneous heterozygous UGT1A1*28 and UGT1A1*6 polymorphisms may produce higher exposure to SN-38 and a higher risk of adverse effects related to irinote-can. Additional studies will be necessary to determine the optimal starting dose of irinotecan for patients with both UGT1A1*28 and UGT1A1*6 polymorphisms. Many studies have demonstrated the impact of UGT1A1 on toxicity of irinotecan. In particular, patients bear-ing UGT1A1 * 28 (TA 7/7) have a higher risk of severe neutropenia and diarrhea. Based on this, prescribers of irinotecan are advised that patients with UGT1A1 * 28 (TA 7/7) should start with a reduced dose of irinotecan, although a particular dose is not specified. Research in Asian countries has shown a lower incidence of UG-T1A1 * 28 (TA 7/7), while UGT1A1 * 6 (A / A) is more often found and is associated with severe irinotecan-related neutropenia. We report here a case of a metastatic colorectal cancer patient who is heterozygous for the UGT1A1 * 28 polymorphism (TA 6/7) as well as the UG-T1A1 * 6 polymorphism (G / A). The patient was treated with FOLFIRI for 9 cycles and underwent two irinote-can dose reductions according to pharmacokinetic data regarding exposure to the active metabolite, SN-38. Simultaneous heterozygous UGT1A1 * 28 and UGT1A1 * 6 polymorphisms may produce higher exposure to SN -38 and a higher risk of adverse effects related to irinote-can. Additional studies will be necessary to determine the optimal starting dose of irinotecan for patients with both UGT1A1 * 28 and UGT1A1 * 6 polymorphisms.