INTRODUCTION: This study describes the development of an instrument to measure the ability of medical students to reflect on their performance in medical practice. METHODS: A total of 195 Year 4 medical students attending a 9-hour clinical ethics course filled in a semi-structured questionnaire consisting of reflection-evoking case vignettes. Two independent raters scored their answers. Respondents were scored on a 10-point scale for overall reflection score and on a scale of 0-2 for the extent to which they mentioned a series of perspectives in their reflections. We analysed the distribution of scores, the internal validity and the effect of being pre-tested with an alternate form of the test on the scores. The relationships between overall reflection score and perspective score, and between overall reflection score and gender, career preference and work experience were also calculated. RESULTS: The interrater reliability was sufficient. The range of scores on overall reflection was large (1-10), with a mean reflection score of 4.5-4.7 for each case vignette. This means that only 1 or 2 perspectives were mentioned, and hardly any weighing of perspectives took place. The values over the 2 measurements were comparable and were strongly related. Women had slightly higher scores than men, as had students with work experience in health care, arid students considering general practice as a career. CONCLUSIONS: Reflection in medical practice can be measured using this semistructured questionnaire built on case vignettes. The mean score allows for the measurement of improvement by future educational efforts. The wide range of individual differences allows for comparisons between groups. The differences found between groups of students were as expected and support the validity of the instrument.