We found an inverse relation between p15 antisense (p15AS) and p15 sense expression in leukaemia. A p15AS expression construct induced p15 silencing in cis and in trans through heterochromatin formation but not DNA methylation; the silencing persisted after p15AS was turned off, although methylation and heterochromatin inhibitors reversed this process. Moreover, 11 out of 16 patient samples (69%) showed relatively increased expression of p15AS and downregulated p15 expression (6/11 in acute myeloid leukaemia and 5/5 in acute lymphoblastic leukaemia). In contrast, 16 normal controls showed high expression of p15 but relatively low expression of the p15AS. Additionally, the two acute myeloid leukaemia lines, which displayed high p15AS and low p15 expression.