A series of sulfonamide hydroxamic acids and anilides have been synthesized and studied as histone deacetylase (HDAC) inhibitors that can induce hyperacetylation of histones in human cancer cells. The inhibition of HDAC activity represents a novel approach for intervening in cell cycle regulation. The lead candidates were screened in a panel of human tumor and normal cell lines. They selectively inhibit proliferation, cause cell cycle blocks, and induce apoptosis in human cancer cells but not in normal cells. The structure-activity relationships, the antiproliferative activity, and the in vivo efficacy are described.