Intersectoral partnerships have been identified as a useful mechanism for addressing the health challenges that face society. In theory, partnerships achieve synergistic outcomes that amount to more than can be achieved by individual partners working on their own. This study aimed to identify key factors that influence health promotion partnership synergy. Data were collected from 337 partners in 40 health promotion partnerships using a postal survey. The questionnaire incorporated a number of multidimensional scales designed to assess the contribution of factors that influence partnership synergy. New validated scales were developed for synergy, trust, mistrust and power. Pearson's correlations and multiple regression analysis were used to identify the significance of each factor to partnership synergy. Trust, leadership and efficiency were shown to be the most important predictors of partnership synergy. Synergy is predicated on trust and leadership. Trust-building mechanisms need to be built into the partnership forming stage and this trust needs to be sustained throughout the collaborative process. We need to develop systems where the best leaders are put forward for intersectoral partnerships. This should be consistent across all sectors and organizations.