In a randomized block experiment sour cherry trees were infected at 1, 2, 4, or 6 years of age with either the necrotic ring spot virus (NRSV) or the sour cherry yellows virus (SCYV) or with both. Tree growth was retarded by both viruses but the effects of SCYV were most severe. A marked retardation of the growth rate following SCYV infection persisted for two to five years depending on the age of the tree when infected. The growth rate of trees infected with both viruses was very similar to that for trees with SCYV only and indicates the predominance of this virus in these combinations. NRSV alone caused a slight reduction in growth rate but there was never an abrupt effect.Because the effects of SCYV on growth and tree form were severe, yields were very low. NRSV caused only a 10 to 30% reduction in growth but the reduction in yield varied from 36 to 56%. Hence the effects of NRS may be of more economic importance than the relatively minor reductions in growth have indicated.