The human gastric pathogen Helicobacter pylori (Hp) has been considered a microaerophile. However, we recently reported that, when supplied with 10% CO2, Hp growth is stimulated by an atmospheric level of O2, suggesting that Hp is a capnophilic aerobe. In this study, we investigated the effects of aerobic O2 tension on Hp cells by comparing gene expression profiles of cultures grown under microaerobic and aerobic conditions in the presence of 10% CO2. The results showed that overall differences in gene expression in Hp cells grown under the two O2 conditions were predominantly growth-phase-dependent. At 6 h, numerous genes were down-regulated under the aerobic condition, accounting for our previous observation that Hp growth was retarded under this condition. At 36 h, however, diverse groups of genes involved in energy metabolism, cellular processes, transport, and cell envelope synthesis were highly up- or down-regulated under the aerobic condition, indicating a progression of the cultures from the log phase to the stationary phase. The expression of several oxidative stress-associated genes including tagD, katA, and rocF was induced in response to aerobic O2 level, whereas trxA, trxB, and ahpC remained unchanged. Altogether, these data demonstrate that aerobic O2 tension is not detrimental to Hp cells but stimulates Hp growth, supporting our previous finding that Hp may be an aerobic bacterium that requires a high CO2 level for its growth.