Little research has been done in Brazilian medical education on the evaluation of clinical reasoning in situations of uncertainty. The most common tests are still multiple-choice, which are capable of evaluating skills when dealing with well-defined problems. However, in practice the majority of situations involve uncertainties. A method for the evaluation of clinical reasoning in contexts of uncertainty was developed on the basis of the cognitive script theory in relation to professional reasoning. The objectives of the research were to develop, apply, and analyze this methodology in a Brazilian educational setting, based on clinical situations in Geriatrics that involved diagnostic, therapeutic, or ethical dilemmas. A group of specialists in this area and a group of undergraduate students that were completing their training in the Geriatrics internship took the test. Comparison of the results led to evidence of the instrument's validity, capable of distinguishing clinical reasoning according to the participants' level of experience. The mean score for the specialists (80,41) was higher than that of students (70,71) (p < 0,001). In addition, analyses of the internal consistency and a G study design furnished results that are consistent with a scoring system that seeks to evaluate a professional skill. In conclusion, a proposal for a script concordance test in the Portuguese language, applied in a Brazilian teaching institution, may be a viable alternative for evaluating clinical reasoning in contexts of uncertainty.