The transcription profiles of four Helicobacter pylori clinical isolates (two cag-negative and two cag-positive) were compared in stationary growth phase using a cDNA-macroarray. The correlation coefficient value between total transcription profiles of clinical isolates H. pylori varied from 0.70 to 0.83. For 44 groups of genes (total number 66) belonging to various functional classes of H. pylori, the correlation coefficient value between these isolates exceeded 0.7, and for 14 groups the value exceeded 0.9. These groups included genes encoding components involved in cell division, adaptations to atypical conditions, electron transport, salvage of nucleosides and nucleotides, glycolysis/gluconeogenesis, folding and stabilization of proteins, translation factors, anaerobic metabolism, and amino acids and amine metabolism. Expression of 52 genes significantly differed between H. pylori clinical isolates. Some of these genes determine microorganism virulence. They include: cytotoxin-associated gene (cagA), genes encoding neutrophil-activating protein (napA), major flagellar protein (flaA), and vacuolizing cytotoxin (vacA), some genes encoding outer membrane proteins (omp), urease alpha and beta subunits (ureA and ureB), and some regulatory proteins, and genes encoding stress-related proteins, such as the chaperone and heat shock protein genes (groEL and dnaK).