Epidermal growth factor receptor (EGFR) signaling contributes to aortic valve development in mice. Because developmental phenotypes in Egfr-null mice are dependent on genetic background, the hypomorphic Egfr(wa2) allele was made congenic on C57BL/6J (B6) and 129S1/SvImJ (129) backgrounds and used to identify the underlying cellular cause of EGFR-related aortic valve abnormalities. Egfr(wa2/wa2) mice on both genetic backgrounds develop aortic valve hyperplasia. Many B6-Egfr(wa2/wa2) mice die before weaning, and those surviving to 3 mo of age or older develop severe left ventricular hypertrophy and heart failure. The cardiac phenotype was accompanied by significantly thicker aortic cusps and larger transvalvular gradients in B6-Egfr(wa2/wa2) mice compared with heterozygous controls and age-matched Egfr(wa2) homozygous mice on either 129 or B6129F1 backgrounds. Histological analysis revealed cellular changes in B6-Egfr(wa2/wa2) aortic valves underlying elevated pressure gradients and progression to heart failure, including increased cellular proliferation, ectopic cartilage formation, extensive calcification, and inflammatory infiltrate, mimicking changes seen in human calcific aortic stenosis. Despite having congenitally enlarged valves, 129 and B6129F1-Egfr(wa2/wa2) mice have normal lifespans, absence of left ventricular hypertrophy, and normal systolic function. These results show the requirement of EGFR activity for normal valvulogenesis and demonstrate that dominantly acting genetic modifiers curtail pathological changes in congenitally deformed valves. These studies provide a novel model of aortic sclerosis and stenosis and suggest that long-term inhibition of EGFR signaling for cancer therapy may have unexpected consequences on aortic valves in susceptible individuals.