Recently developed cDNA microarray or DNA chip technology allows expression monitoring of expression of hundreds and thousands of genes simultaneously and provides a format for identifying genes as well as changes in their activity. In order to search for changes in gene expression after X irradiation in HL60 cells, cDNA microarray technique was done. In this study, expression of 588 human genes (including oncogenes, tumor suppressor genes, cell cycle regulator genes, intracellular signal transduction modulator genes, apoptosis related genes, transcription factor genes, growth factors and receptor genes, cytokine genes, etc) were analyzed. For cDNA microarray analysis mRNAs were extracted from control and 8 Gy-irradiated HL60 cells. As a result the changes in expression of several genes were observed. This alteration of gene expression was confirmed by reverse transcription-polymerase chain reaction. The expression of heat shock 60 KD protein, c-jun, erythroid differentiation factor, CPP32, myeloid cell nuclear differentiation antigen, MAP kinase-activated protein kinase, interleukin-8, monocyte chemotactic peptide 1 and RANTES genes was increased, but the expression of p55CDC gene was decreased after X irradiation.