Objectives: The case history is an important part of diagnostic reasoning. The patient management problem method has been used in various studies, but may not reflect the actual reasoning process because a list of choices is given to the subjects in advance. This study investigated the contribution of the history to making the correct diagnosis by using clinical case simulation, in which students obtained clinical information by themselves. Methods: A prospective study was conducted. Ninety-four fifth-year medical students from Chiba University who underwent supervised clinical clerkships in 2009 were surveyed. Each student randomly selected 1 of 4 test cases and attempted to make a diagnosis through medical interview, physical examination, and laboratory tests, while the teacher acted as a patient. The student ranked the disease(s) diagnosed at each stage of the process. Diagnostic accuracy rates were compared using analysis of the χ2-test. Results: Sixty students (63.8 made a correct diagnosis, which was based on the history in 43 students (71.7, physical findings in 11 students (18.3, and laboratory data in 6 students (10.0. Compared with students who considered the correct diagnosis in their differential diagnosis after taking a history, students who failed to do so were 5.0 times (95 = 2.5-9.8) more likely to make a final misdiagnosis (χ[sup]2[/sup][sub](1)[/sub] = 30.73; p<0.001). Conclusions: History taking is especially important for making a correct diagnosis when students perform clinical case simulation. To improve the diagnostic reasoning skills, medical students should be trained in methods for inferring the correct diagnosis from the case history.