Previous reports of the influence of UGT1A1 gene polymorphisms on the pharmacokinetics of irinotecan metabolism have not assessed Asian patients treated with FOLFIRI plus bevacizumab for advanced and recurrent colorectal cancer. Twenty-one Japanese colorectal cancer patients received intravenous FOLFIRI (bolus irinotecan, folinic acid, and fluorouracil followed by 46-hour fluorouracil infusion) followed by bevacizumab (5 mg/kg) in Cycle 1. In Cycle 2, patients received bevacizumab followed by FOLFIRI. The regimen was in 2-week cycles. The area under-the-curves ratio (AUC0-last) (Cycle 2/Cycle 1) was determined from plasma concentrations of irinotecan and metabolites (SN-38, SN-38G). Safety, efficacy, and drug-drug interactions were analyzed. Median observation period was 7.8 months; median number of cycles 15. Drug-drug interactions were evaluated in eight patients without irinotecan dose reduction. Mean AUC0-last ratios (with/without bevacizumab) of irinotecan, SN-38, and SN-38G were 0.959, 0.927, and 0.931 respectively. Response rate was 65%; median progression-free survival 16.4 months. Response occurred in four patients with, and nine without, UGT1A1 polymorphism. No significant differences occurred between efficacy, safety, or polymorphism status. This cohort showed no differences in safety or efficacy compared to previous reports. Bevacizumab did not affect the pharmacokinetics of irinotecan and its metabolites, irrespective of UGT1A1 polymorphism status.