Backgrounds: Polymorphisms of OPRM1 A118G and ABCB1 C3435T have been suggested to contribute to inter-individual variability regarding pain sensitivity, opioid usage, tolerance and dependence and incidence of adverse effects in patients with chronic pain. This study aimed to investigate the association of both two polymorphisms with opioid requirements in Chinese patients with cancer pain. Methods: The genotypes of rs1799971 (OPRM1) and rs1045642 (ABCB1) were determined by PCR-RFLP and direct sequencing methods respectively in 112 patients with cancer-related pain. Comparisons between the different genotype or allele groups were performed with t-tests or one-way ANOVA tests, as appropriate. The potential relationship of allele number with opioid response was performed with a trend Jonckheere-Terpstra test. Results: In the 112 subjects, the frequencies of variant 118 G and 3435T allele were 38.4% and 37.9%, respectively. Significant higher 24h-opioid doses were observed in patients with GG (P=0.0004) and AG + GG (P=0.005) genotypes than the AA carriers. The dominant mutant 118G allele tended to be associated with progressively increasing 24h-opioiddoses (P=0.001). Compared with CC/CT, patients with ABCB1 TT genotype received higher 24h- and weight-surface area-adjusted-24h- opioids doses (P=0.057 and 0.028, respectively). Conclusions: The OPRM1 A118G single nucleotide polymorphism (SNP) is a key contributor for the inter-individual variability in opioidrequirements in Chinese cancer pain patients. This may possibly extend to the ABCB1 C3435T SNP.