Studies of classroom behavior and learning outcomes have demonstrated that student discussion leads to better learner outcomes. Do these effects, which are based on studies of face-to-face interaction, transfer to the virtual classroom? Existing studies of online postings in asynchronous discussion forums have primarily studied the effects on the authors of the postings. The effect on the recipients--the students reading the postings--has been largely neglected. We set out to fill this gap in our understanding of online discussions by analyzing the effect of reading behavior on course performance. We first perform an in-depth analysis of student discussion behavior in two online courses. We consider the quantity and quality of student postings, as well as the number of postings read by each student. We then test our main findings using a larger dataset of 279 students from eight additional classes, controlling for student grade point average, major, class standing, race, gender, and instructor. Overall, our results suggest that reading is significantly and independently related to course performance. We conclude that discussions have an important place in online classes. (Contains 3 tables and 6 notes.)