The mechanisms of intrinsic and/or acquired anti-cancer drug resistance have been described in in vitro resistance models, but the clinical relevance has remained undefined. We undertook a prospective study to identify correlations between gene expression and clinical resistance to 5-FU/cisplatin. We compared expression profiles from gastric cancer endoscopic biopsy specimens obtained at a chemosensitive state (partial remission after 5-FU/cisplatin) with those obtained at a refractory state (disease progression), using Affymetrix oligonucleotide microarray technology (U133A). Using 119 discriminating probes and a cross-validation approach, we were able to correctly identify the chemo-responsiveness of 7 pairs of training samples and 1 independent test pair. These exploratory data demonstrate that the gene expression profiles differ between chemosensitive and refractory state gastric cancer biopsy samples. (C) 2004 Elsevier Inc. All rights reserved.