Mass customization systems provide consumers with the opportunity to create unique self-designed products. To retailers and manufacturers, the segment of customers configuring unique products is of considerable interest given their potential impact on product diffusion and profits. Yet field evidence suggests that only a minority of consumers use the full potential of such systems by configuring products with unique options (e.g., a volcano red car instead of a white one). The present research shows that the uniqueness of mass-customized products depends on consumer narcissism. Specifically, we demonstrate that (a) consumers higher in trait narcissism configure more unique products (while controlling for self-esteem and need for uniqueness) and (b) state narcissism can be primed via marketing communications to influence product uniqueness. Our findings suggest that firms should consider customers' innate narcissistic tendencies, as well as the ability to influence their current states of mind, to exploit the largely untapped individualization potential of mass customization systems.