Few studies concerning siblings have taken a lifespan approach, i.e., studying sibling relationships from childhood to old age. To outline the developmental changes that occur in the quality of the sibling relationship during various periods of the life cycle and to test the feasibility of devising a coding system to identify such changes, 47 adults were interviewed about the quality of their closest sibling relationship during seven periods of their life cycle. Responses were analyzed according to an eight-category coding system. Results revealed multidimensional developmental changes, such as greater perceived differentiation between siblings early in life and the provision of support during critical life events, such as the death of a parent. The study findings support the feasibility of a coding system to aid in future cross-sectional and/or longitudinal research. (Author/JAC)