Compatible virus infection induces and suppresses host gene expression at the global level. These gene-expression changes are the molecular basis of symptom development and general stress and defence-like responses of the host. To assess transcriptional changes in soybean plants infected with soybean mosaic virus (SMV), the first soybean trifoliate leaf, immediately above the SMV-inoculated unifoliate leaf, was sampled at 7, 14 and 21 days post-inoculation (p.i.) and subjected to microarray analysis. The identified changes in gene expression in soybean leaves with SMV infection at different time points were associated with the observed symptom development. By using stringent selection criteria (>or=2- or <or=-2-fold change and a Q value of <or=0.05), 273 (1.5 %) and 173 (0.9 %) transcripts were identified to be up- and downregulated, respectively, from 18 613 soybean cDNAs on the array. The expression levels of many transcripts encoding proteins for hormone metabolism, cell-wall biogenesis, chloroplast functions and photosynthesis were repressed at 14 days p.i. and were associated with the highest levels of viral RNA in the host cells. A number of transcripts corresponding to genes involved in defence were either downregulated or not affected at the early stages of infection, but upregulated at the late stages, indicating that the plant immune response is not activated until the late time points of infection. Such a delayed defence response may be critical for SMV to establish its systemic infection.