Computer-based clinical simulations for medical education vary widely in structure and format, yet few studies have examined which formats are optimal for particular educational settings. This study is a randomized comparison of the same simulated case in three formats: a "pedagogic" format offering explicit educational support, a "high-fidelity" format attempting to model clinical reasoning in the real world, and a "problem-solving" format that requires students to express specific diagnostic hypotheses. Data were collected from rising third-year medical students using a posttest, attitudinal questionnaire, students' write-ups of the case, and log files of students' progress through the simulation. Student performances on all measures differed significantly by format. In general, students using the pedagogic format were more proficient but less efficient. They acquired more information but were able to do proportionately less with it. The results suggest that the format of computer-based simulations is an important educational variable.