In order to better understand resistance to Phytophthora infestans in tomato, we compared the global gene expression of the susceptible tomato, M82, with its more resistant near-isogenic line, 6-2 (IL6-2), under field conditions using a microarray with more than 12 800 tomato expressed sequence tags (ESTs). Because variance in the field was a major concern, we investigated the likelihood of false positives or false negatives and demonstrated that either probability was very low. The two isolines had indistinguishable constitutive gene expressions prior to inoculation. However, a few genes were particularly prone to variation in both isolines in the absence of P. infestans. Included among these genes were catalase, genes coding for pathogenesis-related proteins, endochitinase and cytochrome P450. In response to inoculation with P. infestans, a time course of gene expression identified 1248 transcripts that were similarly induced or repressed in each line, and 991 that were differentially expressed between the two lines. These differences provide hypotheses to explain the difference in resistance between the two isolines. For example, one hypothesis is that genes up-regulated in IL6-2 in response to inoculation with P. infestans, but not up-regulated in M82, contribute to the resistance in IL6-2. Using virus-induced gene silencing (VIGS), we were able to partially silence two such genes—one encoded a protein with homology to an R gene with the Toll/interleukin-1 receptor–nucleotide-binding site–leucine-rich repeat (TIR-NBS-LRR) motif (37O19) and the other encoded a peroxisomal membrane protein (35P7). Partial silencing of 37O19 reduced the resistance in IL6-2 (P = 0.001), but had no effect on the response of M82. Partial silencing of 35P7 reduced the resistance in IL6-2 moderately significantly (P = 0.067), but had no effect in M82. We expect that hypotheses developed from this gene expression study performed under field conditions will provide an important avenue to an accurate understanding of the genes involved in resistance.