Histonedeacetylase(HDAC)inhibitors such as vorinostat (suberoylanilide hydroxamicacid), valproicacid,romidepsin (FK-228), and LBH589 comprise a relatively new class of potentanti canceragents. This study provides evidence for the potential of vorinostat to cause acquisition of multi drug resistance protein-independent resistance in HCT116 colon tumor cells. This acquired resistance is moderate(two-foldtothree-fold), is non-reversible, and correlates with the loss of responses typically seen with HDAC-inhibitors, that is the loss of acetylation of thehistones H2A, H2B, H3, and H4, the loss of the G2/M checkpoint activation, and the loss of caspase 3-dependent and caspase 7-dependent apoptosis. This acquired resistance also associates with cross-resistance to the hydroxamate-class(LBH589 and JNJ26481585)and to the aliphaticacid-class(valproicacid)HDAC inhibitors but not tothebenzamide-class(MGCD0103)and the cyclic peptide-class(romidepsin) HDAC inhibitors. The acquired HDAC inhibitor resistance described here.