We monitored the number of pairs and the diet of Black-winged Kite Elanus caeruleus on the Lleida Plains (Catalonia, NE Spain) from 1998 to 2012. We examined the diet by means of pellet analysis. Nesting was irregular with peak numbers in some years (2009-2010). We found a positive relationship between the percentage of rodents in the diet and breeding success, an inverse relationship between dietary breadth and breeding success, a negative relationship between dietary breadth and the percentage of rodents in the diet and no significant variation in diet composition or breeding success between low kite population and high kite population years. All of this was consistent with the response of a specialist predator taking advantage of rodent outbreaks. We conclude that the breeding of the Black-winged Kite on the Lleida Plains is more consistent with the existence of irruptive events than with a real colonization of the area.—Llorente-Llurba, E., Bota, G., Pujol-Buxó, E., Bonfil, J., Gálvez, M., Montés, G., Bas, J., Moncasí, F., Pont, F. & Mañosa, S. (2019). Diet composition and breeding success of the Black-winged Kite on the Lleida Plains in relation to population size. Ardeola, 66: 33-50.