Multiple prior studies have examined the nature of news coverage of mammography but have neglected comments generated by readers. However, comments on online news stories have been shown to affect readers’ beliefs and behaviors. Understanding the potential effects of user-generated comments and comments with exemplars, in particular, is necessary to fully understand the effects of online mammography news on media consumers. Study 1 describes the prevalence, content, and representativeness of mammography exemplars in comments on online news about mammography using a content analysis of mammography news articles (n = 71) and comments (n = 5,858) appearing on The New York Times website from November 2009 to December 2014. Study 2 tests the effects of comments on risk perceptions and mammography intentions and mechanisms of these effects using a randomized online experiment with a sample of U.S. women between the ages of 38 and 48. Of comments on news articles about mammography, 31% included a mammography exemplar. Of those, 41% included a mammogram-detected breast cancer exemplar and 19% included a false-positive mammogram exemplar. Additionally, articles with mammography exemplars were more likely to have comments that included mammography exemplars. In Study 2, when compared to comments without exemplars, comments with exemplars did not produce effects on mammography intentions or risk perceptions. Compared to comments about false positives, comments about mammogram-detected breast cancer led to higher intentions to have a mammogram in the next two years, lower intentions to wait until age 50 to have a mammogram, and higher breast cancer risk perceptions. Effects were moderated by participant education level, family history of breast cancer, history of prior mammography, and time spent reading the experimental manipulation. Some effects on mammography intentions were mediated by changes in attitude toward waiting until age 50 to have a mammogram. This study adds to evidence suggesting that comments appearing with news articles have effects on readers. Those who share content online and allow user-generated comments should consider potential effects before allowing comments to be posted.