SUMMARYTwo virus diseases, vein yellows and mosaic, that commonly infect pear trees, reduced the growth and crop of four cultivars (Beurré Hardy, Doyenne du Cornice, Conference and Williams' bon Chrétien) in an experiment continued for 13 years. Reduction in yield was mainly due to the smaller size of infected trees. These produced about 30% less fruit than healthy trees when infected with vein yellows and 40 % less fruit when also infected with mosaic. Fruit size was decreased in only one year. The fertility of Cornice flowers was impaired.The greater vigour of healthy trees did not delay cropping; they produced over 30% more fruit than infected trees in the first three harvest