Monitoring one's own performance accurately is essential for information-processing and self-regulation, which are indispensable in an online learning environment. In this article, the effect of metacognitive monitoring (MM) on interaction in an asynchronous online discussion forum was investigated. Transcripts of this forum, which was integrated in a face-to-face traditional undergraduate course, were analysed in terms of Henri's (1992) cognitive and interactive dimensions. MM was measured by the "cognitive-title" technique, which makes use of the cognitive dimension. Analysis indicated that MM had a large effect size on interaction in the asynchronous online discussion forum. The study particularly suggests that labelling messages with a "cognitive-title" should be a rule of asynchronous online discussion forum protocol. Moreover, this activity might also be a useful tool for improving MM in a virtual environment. (Contains 5 tables.)