Background: Epilepsy is common in people with intellectual disability, yet clinicians and researchers seldom obtain information directly from the client. The development and preliminary validation of a novel measure for use with people with mild to moderate intellectual disabilities is described. Methods: Focus group methods (6 groups; 24 participants) identified issues of concern, and qualitative analysis (NUD*IST) was applied to derive items and themes for a draft scale. Psychometric scale development techniques were then used in a pilot study and subsequent field-testing to investigate validity and reliability (n=46). Results: A total of 148 issues of concern was reduced initially to 52 and then to 42 items using these methods. The derived scale comprised sub-scales reflecting (1) concerns about having seizures; (2) about injury; (3) about issues during; and (4) after seizures; (5) about medication; (6) about what people think; and (7) about daily life. Cronbach for the Glasgow Epilepsy Outcome Scale-Client version (GEOS-C) was 0.92, and ranged from 0.64-0.81 for the sub-scales. Relatively weak associations (r is less than or equal to 0.40), between client and family carer, staff carer or clinician views, suggests that proxy reports are not good predictors of how people with epilepsy themselves are feeling. Preliminary validation suggests that the GEOS-C can discriminate on variables of clinical importance. Conclusions: The GEOS-C complements existing GEOS measures, can be completed in 5-15 min depending upon the level of support required, and may provide a valuable clinical and research tool. Further validational work and appraisal of sensitivity are required.