Public Significance Statement The coronavirus pandemic has been accompanied by a sharp increase in the quantity of misinformation spreading through social media. This "fake news" is often assumed to have terrible consequences for health behavior, however, no studies to date have directly assessed the effect of specific fake news stories on the targeted behavior. In this study, we created novel fake news stories, suggesting for example, that certain foods might help protect against COVID-19, or that a forthcoming vaccine might not be safe. A single exposure to these fabricated stories resulted in small effects on intentions to engage in some of the behaviors targeted by the stories. For example, we found that participants who saw a fabricated story about privacy concerns with a national contact tracing app were 5% less willing to download the app. We also examined whether providing a general warning about fake news might reduce susceptibility, but found no effects on responses to the fake stories. This suggests that generic warnings such as those often used by governments and social media companies are unlikely to be effective in combating online misinformation. These results are important in our efforts to quantify the real-world effects of fake news and reduce its harms.Previous research has argued that fake news may have grave consequences for health behavior, but surprisingly, no empirical data have been provided to support this assumption. This issue takes on new urgency in the context of the coronavirus pandemic, and the accompanying wave of online misinformation. In this large preregistered study (N = 3,746), we investigated the effect of a single exposure to fabricated news stories about COVID-19 on related behavioral intentions. We observed small but measurable effects on some behavioral intentions but not others-for example, participants who read a story about problems with a forthcoming contact-tracing app reported a 5% reduction in willingness to download the app. These data suggest that one-off fake news exposure may have behavioral consequences, though the effects are not large. We also found no effects of providing a general warning about the dangers of online misinformation on response to the fake stories, regardless of the framing of the warning in positive or negative terms. This suggests that generic warnings about online misinformation, such as those used by governments and social media companies, are unlikely to be effective. We conclude with a call for more empirical research on the real-world consequences of fake news.