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Despite the rapid penetration of social media in modern life, there has been limited research conducted on whether social media serves as a credible source of health information. In this study, we propose to identify colorectal cancer information on Twitter and assess its informational credibility.We collected Twitter messages containing colorectal cancer-related keywords, over a 3-month period. A review of sample tweets yielded content and user categorization schemes. The results of the sample analysis were applied to classify all collected tweets and users, using a machine learning technique. The credibility of the information in the sampled tweets was evaluated.A total of 76,119 tweets were analyzed. Individual users authored the majority of tweets (n = 68,982, 90.6%). They mostly tweeted about news articles/research (n = 16,761, 22.0%) and risk/prevention (n = 14,767, 19.4%). Medical professional users generated only 2.0% of total tweets (n = 1509), and medical institutions rarely tweeted (n = 417, 0.6%). Organizations tended to tweet more about information than did individuals (85.2% vs 63.1%; P < 0.001). Credibility analysis of medically relevant sample tweets revealed that most were medically correct (n = 1763, 84.5%). Among those, more frequently retweeted tweets contained more medically correct information than randomly selected tweets (90.7% vs 83.2%; P < 0.01).Our results demonstrate an interest in and an engagement with colorectal cancer information from a large number and variety of users. Coupled with the Internet's potential to increase social support, Twitter may contribute to enhancing public health and empowering users, when used with proper caution.