Objective To develop and validate a self-rating instrument to assess teaching styles among tutors in problem-based learning (PBL). Methods The development of the teaching style inventory (TSI) was based theoretically on four types of teaching behaviours: the assertive, suggestive, collaborative and facilitative styles, as proposed by Bibace et al. A 35-item questionnaire was generated and evaluated for content validity by a group of experienced tutors. The questionnaire was mailed to 196 tutors at the National Taiwan University College of Medicine. The results were submitted for item analysis, internal consistency testing and exploratory factor analysis. Longterm test-retest reliability was assessed by a sample of 50 tutors after a 6-month interval. Results Finally, 118 tutors returned the questionnaires. In the item reduction process, seven items were excluded due to low interscale correlation. Principle component factoring yielded a three-factor solution that accounted for 48·5% of the total variance. Internal consistency coefficients of the four hypothetical domains ranged from 0.73 to 0.83. All domains correlated to each other as expected. Assertive and facilitative styles, which are theoretically opposite teaching styles, showed a negative correlation with each other. Most of the items of each hypothetical domain correlated better with their own domain than with other domains. Longterm test-retest correlations of the four domains ranged from 0.54 to 0.81. Conclusion The TSI demonstrated high internal consistency reliability, acceptable longterm test-retest reliability, and construct validity. Further psychometric testing should focus on applicability to other populations, predictive validity and short-term test-retest reliability. This instrument can be used by programme directors for the recruitment of tutors and can also be used to increase the self-awareness of tutors.