Cucumber mosaic virus (CMV) is widely spread worldwide, causing typical systemic mosaic and other symptoms in tomato (Solanum lycopersicum). Host responses to CMV and molecular mechanisms associated with the development of disease symptoms caused by this virus in tomato are largely unexplored. To investigate plant responses activated during this interaction, we used microarray analysis to monitor changes in host gene expression during disease development. Compared with genes from mock-inoculated control plants, seedlings to adults, 214 of the 3313 tomato genes represented on the array were differentially expressed in CMV-infected plants. Functional classification of CMV-responsive genes revealed that CMV activated typical basal defense responses in the host during the infection process, including induction of defense-related genes, production and scavenging of free oxygen radicals, and hormone synthesis. CMV infection also suppressed a subset of host genes involved in photosynthesis and metabolism. Our results indicate that a wide range of genes play an important role in regulation of the tomato susceptibility response to CMV.