We examine markets where, in addition to production of standard (core) products, mass customization is technologically feasible. We compare a setting where a monopolist provides both standard and custom goods to a setting where an entrant joins the custom market and find customers’ tastes affect the social desirability of entry. The entrant is unconcerned about the impact of its custom production on the incumbent’s core product market and in some cases may supply more custom products than is socially desirable. Entry enhances (reduces) social welfare if consumers’ core valuations are positively (negatively) related to their value added for customization.