The epidermal growth factor receptor (EGFR) plays a prominent role in cell growth and development. Its regulation in humans is complex and incompletely understood. In this study, 12 new polymorphisms were discovered in the 5'-regulatory region of EGFR gene and 2 common single nucleotide polymorphisms (-216G/T and -191C/A) were found in the essential promoter area, one of which is located in a Sp1 recognition site (-216). Transient transfection in human cancer and primary cell lines showed significantly different promoter activity between the two most common haplotypes (-216G-191C and -216T-191C). The replacement of G by T at position -216 increases the promoter activity by 30%. A transient transfection assay in the Sp1-deficient cell line (Schneider cell line 2) showed a strong dependence of EGFR promoter activity on Sp1 and confirmed the effect of the aforementioned polymorphisms. Electrophoretic mobility shift assay also showed a significantly higher binding efficiency of nuclear protein or pure Sp1 protein to the T allele compared with the G allele. We then investigated the allelic imbalance of EGFR transcription in fibroblast cell lines with heterozygous genotype at -216G/T but C/C homozygous genotype at -191C/A. The expression of mRNA carrying T-C haplotype was significantly stronger compared with that of G-C haplotype (P < 0.02). Thus, we successfully showed that a common polymorphism in the EGFR promoter was associated with altered promoter activity and gene expression both in vitro and in vivo. Our findings have implications for cancer etiology and therapy and may also be relevant to the inherited susceptibility of other common diseases.