This study investigated the effect of peer leadership on students' cognitive achievement in online asynchronous discussions. Undergraduate students from a large online course were randomly assigned into groups for four asynchronous discussions, with each group having two to three peer leaders. Results of the mixed-method analysis revealed that the quality of student leaders' lower-order cognitive achievement in online discussion was significantly better than that of student responders. Student leaders' higher-order cognitive level achievement in online discussion had at least the same quality as that of student responders. Moreover, both students' higher-order and lower-order cognitive achievement in discussions had positive and moderate correlation with students' course achievement. Further study should focus on methods to maximize student responders' cognitive achievement in online peer-led discussions. (Contains 3 tables and 1 figure.)