Young adults with mental illness who need continuing care when they turn 18 are referred from child and adolescent psychiatry to general adult psychiatry. During this process, young adults are undergoing multiple transitions as they come of age while they transfer to another unit in healthcare. The aim of this study was to explore expectations and experiences of transition from child and adolescent psychiatry to general adult psychiatry as narrated by young adults and relatives. Individual interviews were conducted with three young adults and six relatives and analysed according to grounded theory. The analysis resulted in a core category: managing transition with support, and three categories: being of age but not mature, walking out of security and into uncertainty, and feeling omitted and handling concerns. The young adults’ and relatives' main concerns were that they might be left out and feel uncertainty about the new situation during the transition process. To facilitate the transition process, individual care planning is needed. It is essential that young adults and relatives are participating in the process to be prepared for the changes and achieve a successful transition. Knowledge about the simultaneous processes seems to be an important issue for facilitating transition.