SUMMARY The shoot epidermis of plants is of prime importance for host and non-host defence against a large number of fungal diseases, including powdery mildew of barley, caused by Blumeria graminis (DC.) E.O. Speer f.sp. hordei. In order to understand better the mechanisms within the epidermis leading to susceptibility as well as durable host resistance, we characterized the transcriptome of two Blumeria-attacked, near-isogenic barley lines of cv Ingrid differing in the presence or absence of the mlo5 resistance gene. A cDNA array was established containing 3128 unique sequences from pathogen-attacked, resistant barley epidermis, which was hybridized with cDNA probes from pathogen-challenged epidermis. Expression analysis resulted in the identification of 311 candidate genes that were differentially expressed in a reproducible manner between control and inoculated epidermis. Among the up-regulated genes were 18 of fungal origin. The amplitude of differential host gene expression was generally higher in the presence of the mlo5 resistance gene as compared with a susceptible interaction. This suggests that mlo-mediated resistance is based on multiple defence compounds and mechanisms once pathogen-derived molecular patterns have been recognized, which yields a possible explanation for the durability of the phenomenon in the field.