Purpose: The need to develop effective tools to measure professionalism continues to challenge medical educators; thus, as a follow-up to a recent examination of the “Conscientiousness Index” (CI, a novel measure of one facet of professionalism) in one setting with preclinical medical students, the authors aimed to investigate the validity of the CI as a proxy measure of professionalism in a different context and in the clinical phase of undergraduate medical education. Method: In academic year 2009–2010, the authors collected data similar to those collected for the original preclinical study. In an effort to create a Clinical Conscientiousness Index (CCI) score, they collected the following information on 124 third-year medical students completing their clinical rotations: attendance, timeliness of assessment submissions, and completion of rotation evaluations. Then, they compared the resultant CCI scores with faculty views on professionalism and with formal assessments of students’ professionalism (i.e., their portfolios and objective structured clinical examinations [OSCEs]). Results: The authors demonstrate significant correlations between CCI scores and faculty views on professionalism (rS = 0.3; P = .001), and between CCI scores and OSCE score (rS = 0.237; P = .008), but not between CCI scores and portfolio assessment (rS = 0.084; P = .354). The authors also present relationships between CCI scores and demographics. Conclusion: The CCI is a practical, valid proxy measure of professionalism, achieving good correlation with faculty views on professionalism and clinical competency examinations, but not portfolio assessment, in one clinical undergraduate setting.