To elucidate the biological dysregulation underlying two forms of inflammatory bowel disease (IBD), ulcerative colitis (UC) and Crohn's disease (CD), we examined global gene expression profiles of inflamed colonic tissue using DNA microarrays. Our results identified several genes with altered expression not previously linked to IBD. In addition to the expected upregulation of various cytokine and chemokine genes, novel immune function-related genes such as IGHG3, IGLL2 and CD74, inflammation-related lipocalins HNL and NGAL, and proliferation-related GRO genes were over-expressed in UC. Certain cancer-related genes such as DD96, DRAL and MXI1 were differentially expressed only in UC. Other genes over-expressed in both UC and CD included the REG gene family and the calcium-binding S100 protein genes S100A9 and S100P. The natural antimicrobial defensin DEFA5 and DEFA6 genes were particularly over-expressed in CD. Overall, significant differences in the expression profiles of 170 genes identified UC and CD as distinct molecular entities. The genomic map locations of the dysregulated genes may identify novel candidates for UC and CD genetic susceptibility.