Wilson disease (WD) is characterized by remarkable variety in its phenotypic presentation. Patients with WD can present with hepatic, neurologic, and psychiatric symptoms combined in different and unpredictable ways. Importantly, no convincing phenotype–genotype correlation has ever been identified, opening the possibility that other genes, aside from ATPase copper-transporting beta (ATP7B), are involved in the pathogenesis of this condition. In addition, modifier genes, or genes that can affect the expression of other genes, may be involved. Clinical and basic science data indicate that environmental and dietary factors can potentially modify gene expression in WD and, consequently, its clinical presentation and course. In particular, previously studied genes include copper metabolism domain-containing 1 (COMMD1), antioxidant 1 copper chaperone (ATOX1), X-linked inhibitor of apoptosis (XIAP), apolipoprotein E (APOE), hemochromatosis (HFE), and 5,10-methylenetetrahydrofolate reductase (MTHFR). Dietary factors include iron and methyl group donors which could affect methionine metabolism and epigenetic mechanisms of gene expression regulation. Most of the work conducted in this field is in its initial stages but it has the potential to change the diagnosis and treatment of WD.