Accumulating evidence suggests that the acetylation and deacetylation of histones play significant roles in transcriptional regulation of eukaryotic cells. The balance between acetylation and deacetylation is an important factor in regulating gene expression and is thus linked to the control of cell fate. The histone deacetylase inhibitors (HDIs) including the hydroxamic acids, such as suberoylanilide hydroxamic acid and pyroxamide, the benzamides MS-275 and CI-994 and the butyrate derivative 4-PBA are a new class of anti-neoplastic agents currently being evaluated in clinical trials. Moreover, new synthetic HDIs have been used recently in phase I and II clinical trials. Over the next few years experts believe that as first generation HDIs produce clinical benefits and second generation inhibitors are rationally designed with improved specificity, this class of drugs will emerge as a new way of cancer treatment. The first clinical studies have shown that histone hyperacetylation can be achieved safely in humans and that treatment of cancer with such agents seems to become possible. The use of HDIs, probably in association with classical chemotherapy drugs or in combination with DNA-demethylating agents, could be promising for cancer patients. Further evaluation is needed to establish the clinical activity of combination therapy using HDIs with cytotoxic drugs or differentiation induced agents.