Introduction: Exposure to residential radon is a preventable cause of cancer. Prevention requires testing, but the percentage of homes that have been tested is small. One reason for the low testing rates may be that printed brochures fail to motivate people to obtain and return a radon test. Methods: We developed a radon app for smartphones that contained the same information as printed brochures. We conducted a randomized, controlled trial that compared the app to brochures in a population comprised largely of homeowners. Cognitive endpoints included radon knowledge, attitudes toward testing, perceived severity and susceptibility to radon, and response and self-efficacy. Behavioral endpoints were participants’ requests for a free radon test and the return of the test to the lab. Participants (N = 116) were residents of Grand Forks, North Dakota, a city with one of the nation’s highest radon levels. Data were analyzed by general linear models and logistic regression. Results: Participants in both conditions showed significant increases in radon knowledge (p< 0.001), perceived susceptibility (p< 0.001), and self-efficacy (p= 0.004). There was a significant interaction, with app users showing greater increases. After controlling for income, app users were three times more likely to request a free radon test. However, contrary to expectation, app users were 70% less likely to return it to the lab (p< 0.01). Conclusions: Our findings confirm the superiority of smartphones in stimulating radon test requests. We speculate that the advantage of brochures in promoting test returns may be due to their ability to serve as physical reminders.