Despite the increase in global research on health inequalities, more needs to be done to strengthen efforts to inform local interventions. In this article, we ask what determines the local capacity to engage in research on health inequalities. A bibliometric analysis identified Spain as the 10th highest global contributor to this research field (1966–2015), yet a significant proportion of this production was affiliated to just a few institutions in Barcelona. How and why has the city produced so much health inequalities research over four decades? A realist explanatory case study was conducted to test and refine six causal mechanisms, and identify the contextual conditions that might have activated them to generate this outcome. Data was triangulated from 10 semi-structured interviews, and scientific and grey literature. Our findings confirmed that several of our proposed mechanisms - functioning under certain conditions over time - explain the large volume of health inequalities research in Barcelona. Based on these findings, key lessons from the city's experience are suggested. Further research is needed on this topic.