The purpose of this study was to examine whether functional polymorphisms in the cytochrome P450 (CYP) enzyme genes affect the risk of developing larynx and hypopharynx squamous cell carcinoma (SCC). We investigated CYP1A1, CYP1B1, CYP2E1, and CYP3A4 polymorphisms in 278 patients with laryngeal and hypopharyngeal SCC and 278 control subjects by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP). Subjects with the CYP1A1 3798CC or TC genotype had an odds ratio (OR) of 3.26 (95% confidence interval CI=1.76-6.03) or 1.56 (95% CI=1.06-2.31), compared with those with the TT genotype. An increased risk was also associated with the CYP1A1 462Val/Val genotype (OR=2.39, 95% CI=1.11-5.16), compared with the TT genotype. Haplotype analysis suggested a synergistic effect of these two polymorphisms. A multiplicative joint effect between the CYP1A1 3798T>C polymorphism and smoking was observed. The OR (95% CI) of the TC or CC genotype for nonsmokers and smokers of >20 pack-years were 1.85 (0.99-3.44) or 8.15 (4.35-15.26), respectively (P(trend)<0.05). The CYP1A1 single-nucleotide polymorphisms are associated with an increased risk of developing smoking-related laryngeal and hypopharyngeal SCC in a Han Chinese population.