This paper describes a new infant-based measure for assessing infant attachment to a sibling during a modified strange situation procedure. Research has demonstrated that young siblings often form a special relationship and that they prefer to interact with one another rather than with an unfamiliar peer. Also it has been shown that older siblings can respond appropriately to the distress of an infant. Research on relationships has moved from frequency counts of discrete behaviours towards more global measures of the ways in which behaviour is organized. These have tended to focus upon the older child as caregiver or the influence of the mother-child relationship on child-child interaction during the strange situation. The main findings of previous studies are presented. This paper describes a new method for measuring sibling attachment. Advantages of the new scoring system over previous procedures are that it utilizes ethological attachment theory, it is infant-based, it allows for a variety of infant reactions to reunion with the older sibling, and it focuses on overall patterns of behaviour rather than tallies of frequences.