This study set out to test whether relevant prior knowledge would moderate a positive effect on academic achievement of elaboration during small-group discussion. In a 2 × 2 experimental design, 66 undergraduate students observed a video showing a small-group problem-based discussion about thunder and lightning. In the video, a teacher asked questions to the observing participants. Participants either elaborated by responding to these questions, or did not elaborate, but completed a distraction task after each question. They received either relevant or irrelevant prior knowledge before the discussion. After the discussion, all participants studied a text about thunder and lighting and completed immediate and delayed-recall tests for this text. Elaboration had no main effect on recall, but there was a significant interaction effect between relevant prior knowledge and elaboration. The results suggest that elaboration is helpful for students with more prior knowledge, but harmful for students with less prior knowledge.