Known of since the 1960s, research into the risk has always been disputed on various grounds particularly the lack of any biochemical explanation for the association. Now, the discovery of two genetic mutations, which raises the risk of thrombosis in the Pill users, may also offer an explanation for it.Although rare, cerebral venous thrombosis (a clot in the sinuses draining blood from the brain and cranium) is strongly linked to hormone use in healthy women with those with genetic mutations at an even greater risk. Recently, 90 per cent of previously healthy women with cerebral sinus thrombosis in the Netherlands and Italy were found to use oral contraceptives (BMJ, 1998; 317: 483-84.)