Cultivar mixtures of winter barley and spring barley together with their component monocultures were grown in field trials to assess the effect of combinations on both straw and grain yield. Significant increases in overall grain yield compared with the mean of the component monocultures with the same inputs were recorded for all trials. Significant decreases in rhynchosporium disease severity were recorded for most non-fungicide treated trial treatments too. The size of these responses was often significantly correlated with component number of the mixtures. The amount of straw produced in mixtures was sometimes significantly changed too, but not always in a positive direction and only correlated with increasing mixture component number in two environments. No correlation of straw yield potential of cultivars with performance in mixtures was found. Cultivar x cultivar x environment interactions appeared to affect the relative yield of grain and straw differentially and therefore it was not possible to predict the effect of mixtures on the harvest index.