BACKGROUND:The purpose of this study was to investigate the fit of a hypothesized model of medical students' diagnostic or clinical reasoning skills based on their aptitude for medical school, basic science achievement, and clinical competency measures. METHOD:A total of 589 medical students who received their MD from 1994 to 2002 participated in this study. Confirmatory factor analysis was used to evaluate the fit of theoretical models of clinical reasoning using measures of basic science and clinical knowledge. RESULTS:The results provided support for a three-factor model of medical student performance (Bentler's Comparative Fit Index = .905, standardized root mean squared residual = .054, root mean squared error of approximation = .105). The clinical reasoning skills of medical students were influenced by an independent relationship between latent variables of basic science achievement and clinical competency. CONCLUSION:The findings support a theoretical model of diagnostic or clinical reasoning that treats the basic science and clinical knowledge of medical students as distinct domains.