Using several proxies for seasonal affective disorder (SAD), we find that SAD negatively affects initial public offering (IPO) first trading day return (underpricing) during the 1981-1989 and 1999-2000 periods, but not during the 1990-1998 and 2001-2007 periods. During those periods when SAD affects IPO underpricing, firm exposure mitigates the negative effect of SAD. In addition, over the years 1981-2000, we find a negative effect of SAD on the offer price revision of the IPO during bookbuilding, which suggests premarket investors adjust for SAD effects during the bookbuilding process.