Dexamethasone (DEX) induces apoptosis in lymphocytes, while protecting some cancer cells from apoptosis, by a poorly understood mechanism. In this study, we examined the potential role of the forkhead transcription factor (FOXO3A) in DEX-induced apoptosis. Unphosphorylated FOXO3A, the active form of FOXO3A, can translocate into nucleus and induce apoptosis. In lymphocytes, FOXO3A is upregulated by DEX treatment, while phospho-FOXO3A was downregulated. In several different types of cancer cells, we found that sensitivity to DEX correlated negatively to expression of phospho-FOXO3A. We conclude that DEX might maintain FOXO3A in its unphosphorylated, active form. Knockdown of FOXO3A expression using a small interfering RNA (siRNA) significantly reduces apoptosis in lymphocytes. This study suggests that FOXO3A has a pivotal role in DEX-induced apoptosis. Increased phospho-FOXO3A levels in cancer cells may explain, in part, their resistance to apoptosis. Therefore, FOXO3A may be a potential target for cancer therapy.