This study examined how major health organizations use Twitter for disseminating health information, building relationships, and encouraging actions to improve health. The sampled organizations were the American Heart Association, American Cancer Society, and American Diabetes Association. A content analysis was conducted on 1,583 tweets to examine these organizations' use of Twitter's interactive features and to understand the message functions and topics of their tweets. The numbers of retweets and favorites were also measured as engagement indicators and compared by different message functions. The results revealed that all of the organizations posted original tweets most, but they differed in the degree to which they used the retweet and reply functions. Hashtags and hyperlinks were the most frequently used interactive tools. The majority of the tweets were about organization-related topics, whereas personal health-related tweets represented a relatively small portion of the sample. Followers were most likely to like and retweet personal health action-based messages.