Exhausting endurance exercise has strong effects on the immune system. Changes have been shown in the cellular composition of peripheral blood and in gene expression within those cells. In this study, custom-made cDNA microarrays focused on inflammation were used to analyze gene expression blood cells obtained from eight half-marathon runners before (t0), immediately after (t1) and 24 hours after exercise (t2). The microarrays that were used contained 384 different cDNAs spotted in triplicate. Differentially-regulated gene expression was analyzed using a simple rule-based clustering. Comparing t1 vs. t0, and t2 vs. t0, 36 and 21 sequences respectively, showed a consistent pattern of changes in all eight athletes. Taken together, the pattern of these modified genes can be viewed as a "gene expression fingerprint" for each time point in response to a half marathon. The known and novel genes identified here represent targets for further molecular characterization of the complex reaction of the body to an exhaustive challenge. These data suggest that gene expression fingerprints can serve as a powerful research tool to design novel strategies for diagnosis and treatment of exercise related injury and stress.