When an adult sibling becomes a long-term missing person the experience is confusing and highly distressing for those left behind. This paper is drawn from a larger study that considered the experience of adult siblings of long-term missing people. It briefly explains the study, the significance of sibling relationships and provides an overview of the experience of missingness, as a way of informing support needs. It suggests that missingness is a loss that is ambiguous (and disenfranchised), unexpected and overwhelming. It is a process that takes individuals time to recognise and negotiate. There is a clear need for greater interprofessional recognition of the consequences of someone going missing and greater cooperation in making an adequate response.