This study investigates human dietary patterns and economic trends at the coastal site of Ancón, Peru during the Andean Middle Horizon (550AD–1000AD) using stable isotopic data from 32 individuals buried at the site. δ13C and δ15N results from human bone collagen and δ13C from human tooth enamel and bone carbonate indicate that inhabitants consumed a mixed diet composed primarily of marine protein and C4 resources, with only marginal reliance on C3 foods. Over time, Ancóneros appear to have relied more heavily on C4 resources, particularly maize, despite the fact that the crop could not have been grown locally. These results are notable given that C3 rather than C4 or marine foods dominate the site's archaeological record. These data suggest that Ancón's inhabitants either had access to more fertile land up-valley where maize could be cultivated successfully or that they engaged in trade relationships with their valley neighbours. A third possibility is that increased maize consumption at Ancón during the Middle Horizon resulted from Wari imperial influence and interregional exchange. Comparisons of δ13C values in enamel and bone carbonate from Ancón individuals indicate that δ13Ccarb_enamel values are significantly more positive than δ13Ccarb_bone values. This suggests that the diets of young children were systematically enriched in 13C compared to that of adults, perhaps as a result of nursing activity and/or differential dietary practices among various age groups at the site. Copyright © 2009 John Wiley & Sons, Ltd.