AIMS:This article reports on the development and validity of a Diabetes-specific Illness Representations Questionnaire (DIRQ) to assess all five dimensions of an individual's perception of diabetes, for adolescents with Type 1 diabetes mellitus. METHODS:There were two development studies. Study 1: participants (n = 115) completed a questionnaire assessing perceptions of the consequences of diabetes and the effectiveness of treatment, along with a questionnaire assessing self-care. Study 2: participants (n = 79) completed a questionnaire assessing their identity, timeline and causal beliefs, along with a diabetes self-efficacy and barriers to adherence questionnaire. Subsequently there were two validation studies. Study 3: participants (n = 44 adolescents and 28 parents) completed the DIRQ and questionnaires assessing their self-care and psychological well-being. Glycaemic control was assessed through retrieval of routine assay results. Study 4: participants (n = 70) completed the DIRQ and questionnaires to assess their psychological well-being, with glycaemic control assessed through retrieval of routine assay results. RESULTS:The development studies suggest that the perceived consequences of diabetes consist of two subscales, perceived threat and perceived impact, and provide further support for the distinction between treatment effectiveness to control diabetes and treatment effectiveness to prevent complications. Along with the validation studies, the results indicate that the questionnaire scales showed good internal consistency and construct validity through their associations with measures of self-care and psychological well-being. CONCLUSION:The data reported here suggest that the DIRQ has the potential to be a useful tool for clinical assessment and evaluating the impact of psycho-educational and medical interventions.