Oidium neolycopersici is a causal agent of tomato powdery mildew. In this paper, gene expression profiles were investigated of susceptible, monogenic- and polygenic resistant tomato genotypes in response to O. neolycopersici infection by using cDNA-AFLP. Around 30,000 TDFs (Transcript Derived Fragments), representing approximately 22% of the transcriptome based on in silico estimation, were identified and 887 TDFs were differentially expressed (DE-TDFs) upon inoculation with O. neolycopersici spores. Forty-two percent of the identified DE-TDFs were detected in both the compatible and incompatible interactions, a subset of these were studied for their temporal patterns. All of these common induced DE-TDFs displayed an expression peak at 7 days post incoluation in monogenic resistant response but sustained up-regulation in the susceptible and the polygenic resistant response. While more than half of these common DE-TDFs showed earlier timing in incompatible interactions compared to compatible interaction. Only 2% of the identified DE-TDFs were specific to either the monogenic or the polygenic resistant response. By annotation of the 230 sequenced DE-TDFs we found that 34% of the corresponding transcripts were known to be involved in plant defense, whereas the other transcripts played general roles in signal transduction (11%), regulation (24%), protein synthesis and degradation (11%), energy metabolism (12%) including photosynthesis, photorespiration and respiration.