Summary Collybia fusipes is a common cause of root rot on oak in the north of France. Collybia fusipes basidiomes can be as frequent on oaks in stands where no decline of the trees occurs compared with stands where the decline is chronic. This might be explained by differences in the amount of roots damaged by the parasite. To test that hypothesis, 430 oak trees, Quercus petraea, Quercus robur and Quercus rubra, located in six forests were selected. Half of them showed C. fusipes basidiomes at the trunk base. The association between presence of basidiomes and decline of affected trees depended on the forest. The level of infection of each tree by C. fusipes, as well as the crown appearance, the tree height : diameter at breast height ratio, age and sapwood width were determined. The presence of C. fusipes basidiomes was always associated with significant root infections. The crowns of the trees deteriorated with increasing level of root infection and the decline was severe only when the root damage was heavy. Although the decline of trees that were heavily damaged by C. fusipes was severe in some of the stands, in others, it was only mild, and so the differences in tree decline between the stands could not be attributed solely to differences in root infection severity. Trees damaged by C. fusipes seemed not to be subjected to more competition than their undamaged neighbour as reflected by a similar tree height: diameter at breast height ratio.