Research suggests that the cognitive style of rumination, the activation and recursive rehearsal of cognitive content, represents a key extension to existing cognitive models of emotional distress in illness. Given limitations with existing rumination measures and the absence of a measure specifically for use in the context of illness, we developed the Multidimensional Rumination in Illness Scale (MRIS). The MRIS is designed to incorporate both general and illness-specific dimensions of rumination and meta-cognitive beliefs about rumination. In Study 1 an exploratory factor analysis of an initial pool of 60 items was undertaken. This was followed by confirmation of the hypothesised factor structure in Study 2. In Study 1, participants diagnosed with a chronic physical or mental illness (n=251) completed an online pilot version of the MRIS comprising 60 items generated from an extensive literature review and existing rumination measures. Exploratory factor analysis and parallel analysis were conducted to identify common factors and facilitate item reduction. Factor structures were also examined for consistency across gender and illness type. In Study 2, participants with a chronic physical or mental illness (n=266) completed a revised version of the MRIS comprising 41 items. Confirmatory factor analysis assessed the adequacy of a proposed four factor model of rumination, concurrent and discriminant validity was evaluated, and test-retest reliability examined. In Study 1, exploratory factor analysis of a reduced 36 item scale with oblique rotation suggested a four factor solution for rumination in illness (intrusiveness, brooding, instrumental and searching for meaning). Internal consistency reliability indices were satisfactory. The factor structure was found to be coherent both for gender and health condition. In Study 2, confirmatory factor analysis confirmed the adequacy of the multi-dimensional model of rumination in illness. The MRIS demonstrated good internal consistency, for the full scale and sub-scales, as well as good test-rest reliability over two weeks, and good concurrent and discriminant validity. The MRIS is a reliable and valid measure that should provide a comprehensive assessment of the cognitive style of rumination in the context of both physical and mental illness.