Habitat loss and fragmentation inevitably cause biodiversity decline, a major concern for the conservation of endangered species. Primates are of particular interest, because they are highly vulnerable to forest fragmentation. In this study, we investigated faecal glucocorticoid measurements (FGCM), an indicator of physiological stress, in an endemic and IUCN-endangered monkey species inhabiting forest blocks in the Udzungwa Mountains in south-central Tanzania. The Udzungwa red colobus (Procolobus gordonorum) is threatened by hunting, and habitat degradation and loss due to agricultural expansion. They are highly folivorous, more abundant at lower elevations, but able to persist in extremely small forest blocks. We collected faecal samples in four forest blocks that differed in size, level of human impact (hunting and past logging activities) and ecological factors to examine the potential influence on stress hormone output. Across all four forests, we did not find any clear effect on FGCM levels of the variables considered, apart from elevation. Red colobus had higher FGCM levels at lower elevations, where their seemingly optimal habitat occurs, but concomitantly where greater human disturbance is found. Although our results should be interpreted with caution and confirmed with further study, we suggest that this species is not negatively influenced by a moderate level of human activity, consistent with ecological modelling data.