Given both the importance and difficulty of promoting community-based public health coalitions, their capacity for sustainable action merits systematic examination. The current study addresses this need, focusing specifically on the "relational" dimension of capacity, that is, how relationships both among members and with external actors affect coalition-level activity. The context is a multimethod comparative case study of two rural cancer control coalitions. The authors began by using quantitative and qualitative data to characterize relational capacity in each coalition and then assessed the association between coalition-level relational capacity and level of subsequent interventions. The more active coalition had a more inclusive relational structure than did its less active counterpart but also placed less emphasis on personal friendships. The authors conclude that coalitions' relational structures are measurable and that this dimension of capacity may affect sustainable capacity for health promotion. (Contains 3 tables and 1 note.)