Indices of diet quality are useful tools for research on the ecology, behaviour and the management of wildlife. The quality of diets is an essential driver of the performance of individuals, which in turn drives the dynamics of populations. Dietary nitrogen (DN) is a good index of diet quality in some herbivores, and faecal nitrogen (FN) has proved a good predictor of DN in grazers, but there is some doubt as to its validity in browsers, since they eat food with significant amounts of tannins, which bind with proteins. In order to test the validity of FN to predict DN, we first test for seasonal and spatial variations of FN in a population of roe deer Capreolus capreolus. Secondly, we examine the relation between DN and FN according to the occurrence of condensed tannin in the diets of 41 individuals of this population. Third, using feeding trials with captive roe deer, we test if free condensed tannins in diets affect the relationship between DN and FN. The results show that FN broadly follows seasonal and spatial variations of food quality, with higher values in spring, and higher values in roe deer living in a farmed area with richer food resources compared to those of deer in forests. Both in wild and captive animals, DN increases with FN when the dietary condensed tannin is low or absent, but the relationship disappears when there is an appreciable amount of condensed tannin in the diet (> 2%). We conclude that FN is not suitable for predicting individual short term diet quality for browsers that ingest significant levels of tannin. FN may, nonetheless, reflect broad seasonal and spatial patterns of diet quality at the population level.