While the potentials of virtual worlds to support experiential learning in medical education are well documented, assessment of student learning within these environments is relatively scarce and often incongruent. In this article, a conceptual framework is proposed for formatively assessing dispositional behaviours in scenario-based learning within a virtual world. The framework was devised for use with medical students playing the roles of junior doctors as they solve open-ended clinical cases within an environment called the "Otago Virtual Hospital." Drawing upon Perkins, Jay and Tishman's (1993) dispositional theory of thinking, it is proposed that the assessment of dispositional behaviours in scenario-based activities can be carried out by measuring the number of times students either seize or miss an opportunity to engage in a particular dispositional behaviour. The approach can potentially also be used for assessing scenario-based learning in other disciplines (e.g. law, business, military). (Contains 1 figure and 3 tables.)