Dutch elm disease is a phytopathogenic fungal disease transmitted by bark beetles, and it has eradicated elms from large parts of Europe and North America. I investigated phenotypic differences between diseased and healthy trees before and during infection, with special emphasis on measures of developmental instability of leaves (fluctuating asymmetry, frequency of phenodeviants) that may reflect adverse environmental and genetic conditions during foliar growth. I investigated whether stressed broad-leaved elm, Ulmus glabra, trees, as estimated from their leaf asymmetry and frequency of foliar phenodeviants, were more susceptible to the disease than trees with less asymmetric leaves. Trees that became diseased had larger leaf asymmetry than the nearest healthy neighbouring tree before acquisition of the disease. Foliar asymmetry was also caused by the disease since the degree of asymmetry increased from the first to the second year of infection. The frequency of elm leaves with abnormal morphology increased considerably from trees that were healthy to those that became diseased and particularly to the second year of the infection. Furthermore, the relative frequency of foliar side veins that joined veins from the other half of the leaf at the central vein decreased from healthy trees over susceptible trees that became infected to diseased trees. These results suggest that measures of developmental instability of elm leaves reflected susceptibility to Dutch elm disease as well as the negative effects of the disease on phenotypic development.