In this report we describe investigation of the RASSF family for inactivation by promoter CpG island hypermethylation in a large series of childhood B- and T-ALL and find very frequent promoter hypermethylation and loss of expression of RASSF6 and RASSF10 in B-ALL and T-ALL respectively. Here, RASSF6 is linked to acute lymphoblastic leukemia.