What it is like when a lifelong twin relationship ends through death in later life is the focus of this study. It draws on interview data from seven twins who are part of a longitudinal Swedish twin study (SATSA) and who lost their co-twins in old age. Data were analyzed using qualitative latent content analysis. The results showed that the experience of loss of the co-twin was profound, including an emotional as well as a behavioral dimension. Loss and loneliness were expressed as the dominant feelings related to the quality of the missing relationship as well as the loss of twin identity. However, the grief experiences in this study were primarily related to the closeness and quality of the twin relationship, rather than identity. Behavioral adjustments included the use of outside as well as internal cognitive resources to cope with life after the loss. Despite the devastating experience of losing a co-twin after a lifelong relationship, the participants engaged actively in their own grief processes. It was concluded that twin loss is unique, in the sense of losing the relational twin identity, as well as it is characterized by similar features as the loss of a close relationship among non-twins.