Each year students spend thousands of hours participating in discussion forums. Yet there has not been sufficient scholarship on the observable effects of discussion forums on learning outcomes, and there is not a clear understanding of the role discussion forums play in transmitting knowledge of course concepts to students. This study investigates associations between participation in online discussion forums and learning performance on discussion forum topics by tracking involvement in discussions as well as student mastery of discussion topics. Furthermore, the level of difficulty of discussion topics was evaluated. In some instances, discussion forum participation associated with better learning outcomes, while in other instances no associations were found. Although discussion forums did not aid learning for an easy concept, discussion forums associated with better learning outcomes for students participating in a moderately difficult topic. For a high difficulty topic, participation alone in a discussion forum did not correlate with improved learning outcomes; however, high quality involvement in a discussion forum of a highly difficult concept was associated with improved learning outcomes.