Improvements in the level and quality of lamb output are required to help sustain the financial viability of hill sheep farming. In the lowland sector breed substitution has been shown to be an effective means of increasing ewe productivity and lamb carcass lean content. However, there is a lack of adequate contemporary breed comparisons for the hill sector to base breeding decisions on. Consequently, the objectives of this study were to investigate the effects of ewe and ram breed on lamb output under a range of hill environments.