This paper reports an analysis of the yield of sugar beet crops grown under experimental conditions between 1993 and 1995 in the UK. Crops were either healthy (unstressed) or subjected to drought, infection with Beet yellows virus (BYV) or a combination of both. The study investigated whether the large differences in yield between the crops grown in different seasons and subjected to different stresses could be accounted for by simple relationships between total biomass and radiation interception (εs), transpiration (εw) or εs and εw adjusted for mean saturation deficit (Ωs and Ωw respectively). Mean values of εs, εw, Ωs and Ωw in healthy crops were 1·42 g/MJ, 0·89 g/kg, 6·76 g/kPa/MJ and 4·29 g/kPa/kg respectively. Variations in the dry matter yield between seasons were best accounted for by Ωw and less well by εw or εs. Ωs accounted for least variation in yield between seasons. None of these relationships remained constant in stressed plants; both drought and BYV-infection decreased εs (and Ωs) but Ωw was increased by drought and decreased by BYV-infection. However, in common with healthy crops, seasonal variation in yields was best accounted for by Ωw. Mean values of εs, Ωs, εw and Ωw for all healthy, infected and droughted crops accounted for 61, 50, 88 and 97% of the variation in dry matter yield between experiments respectively. Accurate prediction of the yield of stressed plants requires a knowledge of their infection and drought status. If this information is unavailable then the mean value of Ωw for healthy, infected and droughted crops will provide a reasonable prediction of the yield of such crops.