Wood ants of the Formica rufa group are ubiquitous keystone species in Eurasian boreal forests. They and aphids live in a mutual relation where, in exchange for protection, aphids provide ants with sugary honeydew processed from tree sap. Conifers, unlike deciduous trees, have only few defoliating insects, and the growth loss of conifers due to sap sucking by aphids is thus not compensated by reduced insect herbivory due to predatory ants. The effect of ant-aphid mutualism on tree growth can change with stand age, because forest clear cutting harms wood ants. It was studied whether the mutualism between wood ants and Cinara aphids affects the growth of Norway spruce (Picea abies) in stands of different ages. The study was done in spruce stands of four age classes (5, 30, 60 and 100 years) in eastern Finland. Ten spruces visited heavily and ten spruces visited lightly by ants were selected around five medium-sized ant mounds in each stand age class. The access of ants was blocked to half of the trees in both groups. Only in the 30-year-old stands, the mean annual radial growth of the heavily-visited spruces was significantly 7.3% smaller than in trees where ant traffic was blocked. The results indicate that the ant-aphid mutualism can have a significant effect on the growth of individual spruces, but its effect is negligible at a stand scale.