The Cochrane review (Lethaby et al 2004) and analysis of available observational data (Anderson 2003, Beresford 97, Jain 2000, Hill 2000, Hully 98, MWS 2005, Newcomb 2003, Pike 97, Pukkala 2001, Weiderpass 99) suggest that addition of progestogen for at least 12 days per cycle reduces, but may not completely eliminate, the increased incidence of endometrial cancer caused by unopposed oestrogens; and that combined continuous HRT does not increase the risk of endometrial cancer.