In the present study, the authors examined the long-term effectiveness of a course for residents on how to teach students, patients, and peers. Residents of various specialties attended a mandatory short course on clinical teaching skills in the middle of their first year of postgraduate medical training. Three types of evaluation data were collected at three times during a two-year period: self-ratings by the residents, questionnaires completed by the residents, and ratings completed by students taught by the residents. Complete data for 18 residents indicated that the residents rated their teaching skills significantly higher after the course (at the end of both the first year and the second year) than before it. At the end of the second year, 94 percent of the residents stated that the course was helpful, 67 percent could recall and explain specific principles of teaching, and 61 percent reported using principles from the course in their teaching. Students' ratings of these 18 residents were too scanty to be interpreted validly. The study suggests that residents of varying specialties can profit from an introductory course on teaching skills and that the effects endure for at least one and a half years.