Previous research has argued that the mental well-being of d/Deaf people is poorer than that of hearing populations. However, there is a paucity of valid and reliable mental health instruments in sign language that have been normalised with d/Deaf populations. The aim of this study was to determine the reliability of the Clinical Outcomes in Routine Evaluation – Outcome Measure (CORE-OM) with d/Deaf populations. A British Sign Language (BSL) version was produced using a team approach to forward translation, and a back-translation check. The CORE-OM was incorporated into an online survey, to be completed in either BSL or English, as preferred by the participant. From December 2010 to March 2011, data were collected from 136 d/Deaf people. Cronbach's α was used to measure the internal consistency of items in the CORE-OM. Comparisons were made between versions, including comparisons with the non-clinical hearing population (not in receipt of mental health services) in a previous study. The reliability of the overall score, as well as the non-risk items in both the BSL and English versions, was satisfactory. The internal reliability of each domain in the BSL version was good (Cronbach's α > 0.70) and comparable to the English version in the hearing population. This was true for most domains of the CORE–OM in the English version completed by d/Deaf people, although the Functioning domain had a relatively low α of 0.79 and the Risk domain had an α of only 0.66 This raised the question whether it is advisable to use a mental health assessment with d/Deaf populations that has been standardised with hearing populations. Nevertheless, this study has shown that it is possible to collect data from d/Deaf populations in the UK via the web (both in BSL and English), and an online BSL version of the CORE-OM is recommended for use with Deaf populations in the community.