We studied the diet of Crested Guans (Penelope purpurascens; Galliformes: Cracidae) in lowland tropical forest on Barro Colorado Island, Panama. From June-August 2015 we collected observations of foraging adults using five-minute focal follows of randomly chosen individuals. A total of 141 food items were recorded over 565 minutes of observations. Although Crested Guans are believed to be primarily frugivorous, we found that they consumed a high proportion of leaves (61.7% of items) and insects (24.8%), with fruits comprising a minority of items (13.5%). Guans preferred to eat young leaves, which are more tender and contain higher levels of nitrogen than mature leaves; however, young leaves also contain higher levels of tannins and other secondary defensive compounds, raising new questions about the mechanisms by which arboreal birds digest leaves. Recent studies have found similarly high levels of leaf consumption in other species of guans and chachalacas, suggesting that folivory may be more widespread in the family Cracidae than previously realized.