This study investigated the effects of teacher feedback on changes in the spoken language performance of a group of advanced learners of English in the Netherlands. Participants were medical researchers attending a course entitled English for Medical Congresses. The course included a sequence of speaking tasks and culminated in a conference. Tasks were graded, and feedback was designed to increase in complexity. A combination of written feedback and audio- and videotape recordings were used. The study examined whether students became aware of, and mastered, language problems identified in teacher feedback as well as problems not identified in feedback, and whether they were aware of changes in their language performance. Analysis of data from student surveys, written feedback, language logs, and audio and video recordings indicated that students' perceptions of the value and effect of feedback were matched by real improvements in their spoken performance. The course outline, language log, and questionnaire are appended. (Contains 40 references.) (SM)