Hill sheep flocks in the UK are dominated by purebred ewe genotypes, with the Scottish Blackface being the most common. However a long term decline in economic returns, combined with recent changes to the Common Agricultural Policy, has lead many hill sheep producers to consider keeping crossbred ewes to exploit the benefits of hybrid vigour for lamb survival and to introduce beneficial traits for prolificacy, ease of lambing and carcass quality. In 2001, a major on-farm research programme was initiated to evaluate the performance of a range of crossbred ewe genotypes for the Northern Ireland hill sheep sector. 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). However, ewe longevity is a major issue for hill flocks, where the annual replacement rate often exceeds 20%. Therefore it is inappropriate to evaluate crossbred ewe genotypes based on average annual performance alone, and the aims of this study were to investigate the lifetime performance of a range of crossbred genotypes under hill conditions.