PURPOSE: Medical professionalism is a fundamental competency for all physicians and continuous development of professionalism during residency training is crucial. The purpose of this study was to develop an instrument assessing residents' attitudes toward unprofessional behaviors. METHODS: A questionnaire survey was conducted in cooperation with the Korea Resident Association from May to July 2013. A total of 317 residents from seven university-affiliated hospitals in South Korea participated in the survey. RESULTS: In the exploratory factor analysis, seven factors were extracted from the data; factor loadings of the 44 items ranged between 0.40 and 0.89. Through iterative discussion, three items below 0.45 were deleted and one additional item was removed due to its irrelevance. Twelve items included in Factor 1 were divided into two different categories. A final version of the questionnaire containing 40 items in eight categories was assessed using confirmatory factor analysis. It was deemed to have a good fit; the root mean square error of approximation and comparative fit index were 0.07 and 0.9, respectively. The reliability (Cronbach's ) of the inventory was 0.97. CONCLUSION: The items of this instrument encompass a broad range of residents' behaviors in clinical practice, research, and publication. In addition, it includes some types of misconduct that can be considered unique features of the authors' cultural backgrounds. We recommend this instrument as an assessment tool to diagnose residents' perceptions and attitudes towards professionalism lapses and to provide insight regarding potential improvement in professionalism education.