We investigated whether UGT1A1 polymorphisms are associated with the prognosis of ovarian cancer patients treated with irinotecan. UGT1A1 genotypes were analyzed in 11 stage I ovarian clear cell carcinoma patients who received irinotecan as first-line therapy. Progression-free survival, overall survival and adverse events were also assessed for each genotype. Three patients harbored UGT1A1*1/*6 while another three harbored UGT1A1*1/*28. Two patients with a wild-type genotype experienced recurrence and one died, whereas no recurrence or death was observed in patients with heterozygous genotypes. Adverse events tended to be more severe in patients with UGT1A1*6 and *28, although progression-free survival and overall survival rates tended to be better than in wild-type; the differences were not significant. We conclude that UGT1A1 polymorphisms have the potential to be a prognostic marker of irinotecan treatment.