White blood cells (WBCs) express tens of thousands of genes, whose expression levels are modified by genetic and external factors. The purpose of the present study was to investigate the effects of acute exercise on gene expression profiles (GEPs) of WBCs and to identify suitable genes which may serve as surrogate markers for monitoring exercise and training load. Five male participants performed an exhaustive treadmill test (ET) at 80% of their VO₂max, and a moderate treadmill test (MT) at 60% VO₂max for exactly the same time about two weeks later. WBCs were isolated by the erythrocyte lysis method. GEPs were measured using the Affymetrix GeneChip® technology. After scaling, normalisation, and filtering groupwise comparisons of gene expression intensities were performed and several validated by real-time PCR. We found 450 genes up-/ and 150 down-regulated (> 1.5-fold change; ANOVA with Benjamini-Hochberg correction, p<0.05) after ET which were closely associated with the gene ontology lists "response to stress" and "inflammatory response". Analysis of mean expression levels after MT showed that the extent of up- and down-regulation was workload dependent. The genes for the stress proteins HSPA1A, HSPH1 and the matrix metalloproteinase MMP-9 showed the most prominent increases whereas the YES1 oncogene (YES1) and CD160 (BY55) were most strongly reduced. Despite different methodological approaches used the consistency of our results with the expression data of Connolly et al. (4) suggests that expression fingerprints are useful tools for monitoring exercise and training loads and thereby helps to avoid training associated health risks.