The standard procedure for diagnosing allergic contact dermatitis is to perform a patch test. Because this has several disadvantages, the development of a new in vitro test system would be of immense value. Gene transcripts that distinguish allergics from non-allergics may have the potential to serve as the molecular basis for such a diagnostic tool. In this study, we use the microarray technology in the identification of differentially expressed genes in allergen-stimulated peripheral blood mononuclear cells (PBMCs) from 3 chromium-allergic patients versus 3 healthy controls. Using an Affymetrix GeneChip, the gene expression was analysed in PBMC cultures grown with 100 microg/ml CrCl3 or in media alone for 24 hr. A total of 26 genes were differentially expressed by more than twofold (P < 0.01) in allergen-activated PBMCs from patients compared with controls. 18 of these were upregulated, whereas 8 were downregulated. The expression of 1 downregulated gene, CASP8, was also found specifically and significantly reduced in an expanded population including 4 additional chromium allergic patients and 1 additional control subject by real-time reverse transcriptase polymerase chain reaction (RT-PCR) analysis. The expression of 2 upregulated genes, ETS2 and CISH, correlated with a high-proliferative response following CrCl3 exposure. Additionally, real-time RT-PCR analysis indicated that the same gene expression changes are valid for nickel allergics, potentially making the expression profile more widely available. The 26 differentially expressed genes identified in this study may potentially function as diagnostic markers for contact sensitivity.