I examine the importance of price support regulation in explaining IPO underpricing in the Korean stock market from 2001 through 2007. In contrast to the US practice where price support is provided effectively at the cost of the issuing firms, the price support in Korea resulted in direct costs to the underwriters. I construct a simple model to capture this feature of the regulation where IPO prices are determined through the interaction of the maximizing behaviors of underwriters and issuers. In the model, three variables, namely, the expected post-IPO price volatility, size of newly issued shares, and size of tradable shares, are specified to affect the opportunity costs of underpricing. When combined with the regulatory regime change in 2003, which lightened underwriters’ obligations towards price support, the model implies that the magnitudes of the relationships between the three variables and underpricing have decreased since 2003. I test the hypotheses and find supportive empirical results: the relationship of underpricing with the expected price volatility has changed from positive to insignificant; those with sizes of newly issued and tradable shares from insignificant to negative. The findings contrast with existing Korean studies that do not find any evidence that price support regulation decreased the opportunity cost of underpricing for underwriters. The results also illustrate a more general point that to fully understand underpricing in a given stock market, it is crucial to take into account the regulatory environment that systematically influences agents’ incentives to control or generate underpricing.