Fifteen polymethoxyflavones (PMFs) and hydroxylated PMFs were isolated from sweet orange (Citrus sinensis) peel extract and synthesized to investigate their biological activity. All obtained compounds were tested in HL-60 cancer cell proliferation and apoptosis induction assays. While some PMFs and hydroxylated PMFs had moderate anti-carcinogenic activities, 5-hydroxy-6,7,8,3',4'-pentamethoxyflavone and 5-hydroxy- 3,6,7,8,3',4'-hexamethoxyflavone showed strong inhibitory activities against the proliferation and induced apoptosis of HL-60 cell lines. (c) 2007 Elsevier Ltd. All rights reserved.