Soil application of phenol increases the resistance of elms to Dutch elm disease (DED), but the mechanisms involved in this enhanced resistance are poorly known. Based on the hypothesis that exogenous phenols play a role in the survival of some elm stands to DED, the effects of phenol on (i) mycelial growth and sporulation of Ophiostoma novo-ulmi, (ii) tree phenology, (iii) wood formation, (iv) histochemistry of xylem tissues, and (v) resistance of elms to DED, were studied. Phenol at 0.2% strongly inhibited both mycelial growth and sporulation of O. novo-ulmi. In a greenhouse experiment, trees watered with a 0.2% phenol solution accumulated phenol in distal twigs, and showed a delayed wood formation, suberized tangential bands in their xylem tissues, and a reduction of DED symptoms, in comparison to control trees. Soil texture had no effect on phenol accumulation in xylem tissues or on tree resistance level. The presence of two phenolic glycosides in the phenol-treated elms was interpreted as a mechanism for phenol detoxification. In a semi-natural Spanish elm stand where phenol-based products were applied in the past, phenol was not detected in the soil, probably because of its degradation. In the same stand, healthy trees located in areas with either low or high incidence of DED showed a similar phenol content in their xylem tissues. The role of exogenous phenol in the resistance of some elms to DED is discussed.