In the UK and elsewhere in Europe, high densities of mountain hares Lepus timidus are associated with heather moorland, the area of which has been diminished by large-scale afforestation. The consequences of this landscape change for the diet composition and body weight of mountain hares were investigated by comparing mountain hare carcasses collected in November 1990 simultaneously from a large hare-fenced, young forestry plantation and from the adjacent heather moorland. The sex, age, diet composition and body weight of individual hares were recorded. There were readily identifiable differences in the diet composition and body weight of hares that occupied the two different habitats, the segregation of which had been rigorously maintained for the previous nine months and included the period of births of leverets. Adult females and leverets in the young forestry plantation had a higher proportion of grasses and a lower proportion of heather in their stomachs than the same age/sex classes on the open moorland, and leverets occupying the forestry plantation were significantly heavier than those inhabiting the moorland habitat. Large-scale afforestation results in an improvement in diet quality and body weights in young mountain hares during the early plantation stages, and in those early years following planting, commercial afforestation is probably not detrimental to mountain hare populations. Persistence of mountain hare populations in afforested landscapes could be facilitated by the incorporation of appropriate young age classes of forest.