Histone deacetylase inhibitors have attracted considerable attention because of their ability to overcome the differentiation block in leukemic blasts, an effect achieved either alone or in combination with differentiating agents, such as all-trans retinoic acid. We have previously reported favorable effects of the potent histone deacetylase inhibitor valproic acid in combination with all-trans retinoic acid in patients with advanced acute myeloid leukemia leading to blast cell reduction and improvement of hemoglobin. These effects were accompanied by hypergranulocytosis most likely due to an enhancement of nonleukemic myelopoiesis and the suppression of malignant hematopoiesis rather than enforced differentiation of the leukemic cells. These data prompted us to investigate the effect of valproic acid on normal hematopoietic stem cells (HSC). Here we show that valproic acid increases both proliferation and self-renewal of HSC. It accelerates cell cycle progression of HSC accompanied by a down-regulation of p21(cip-1/waf-1). Furthermore, valproic acid inhibits GSK3beta by phosphorylation on Ser9 accompanied by an activation of the Wnt signaling pathway as well as by an up-regulation of HoxB4, a target gene of Wnt signaling. Both are known to directly stimulate the proliferation of HSC and to expand the HSC pool. In summary, we here show that valproic acid, known to induce differentiation or apoptosis in leukemic blasts, stimulates the proliferation of normal HSC, an effect with a potential effect on its future role in the treatment of acute myeloid leukemia.