This study examined the relevance of adult attachment to the relationships between mental health services and their clients. The aim of the study was to develop a self-report measure with acceptable levels of reliability and validity and which was grounded in the experiences of service users, to assess the ability of adult mental health services to meet clients' attachment needs. A combination of qualitative and quantitative methodology was used. The Service Attachment Questionnaire (SAQ) was developed via focus groups of service users, followed by a grounded theory analysis of the group data to identify themes and potential items for inclusion. Two clinical trials were conducted to assess its reliability and validity. The grounded theory analysis produced six key themes forming the basis of the six-subscale, 25-item SAQ. It demonstrated good levels of internal and test-retest reliability. Factor analysis of the subscales revealed a single underlying construct. The SAQ is a reliable and usable self-report measure, and indicates that attachment is relevant to the relationships clients have with mental health services. As the measure is the first of its kind, further exploration of the measure's reliability and validity is recommended.