Genetic polymorphisms of uridine diphosphate glucuronosyl transferase 1A1 (UGT1A1), and SLCO1B1 coding organic anion-transporter polypeptide 1B1 are independent risk factors known to increase irinotecan toxicity in adults. Although combined occurrence of polymorphisms in these 2 genes is likely to influence susceptibility to irinotecan toxicity, data are scarce, especially in children. We report an 11 year old female with severe and prolonged neutropenia after irinotecan-based chemotherapy. The patient’s genotyping revealed polymorphisms in both UGT1A1 and SLCO1B1. To our knowledge, this is the first case report of combined genotyping of both UGT1A1 and SLCO1B1 in a child with severe irinotecan toxicity.