Hill sheep flocks in the UK are dominated by purebred ewe genotypes, with the Scottish Blackface, Welsh Mountain and Swaledale being the most common. However recent changes to the Common Agricultural Policy have lead many hill producers to consider keeping crossbred ewes on the hill, with the aim of introducing complementary traits to increase lamb output and improve carcass quality. In 2001 a major on-farm research programme was initiated on 6 hill farms around Northern Ireland to evaluate lifetime performance of a range of crossbred ewe genotypes. Provisional data has already identified that retaining Lleyn X Blackface and Texel X Blackface ewes can improve lamb output at weaning by up to 10% relative to purebred Blackface ewes (Speijers et al., 2007). The aims of the current study were to investigate the effects of switching to crossbred hill ewe genotypes on lamb performance during finishing.