A number of factors may affect student interaction in an asynchronous online discussion forum used in learning. This study deals with student preferences for the subject line of messages and in what ways the choice of subject line contributes to coherence and interaction reflected in the textual and interpersonal functions of the linguistic items used. The study also attempts to determine what affects the choices made by participants. Nine separate discussion forums from three different undergraduate courses in English at a Swedish university were used in the study. A total of 98 students and 435 student messages were examined and a number of trends appeared. The functions of the subject line may be summarized as contributing to coherence by reflecting message content in a number of different ways. In addition, the subject line can perform other tasks such as maintaining social relationships among the participants. It is not clear in what ways the subject line contributes to interaction with regard to increasing the reading rate. The trends observed indicate that other factors than subject line content may contribute to whether students are inclined to access a message or not, such as when a message is posted and where it is displayed on the screen. (Contains 5 tables and 2 figures.)