Loss of chromosome Y (LOY) in the bone marrow has long been considered as an age-related phenomenon with an incidence of more than 25% in males beyond the age of 80 years. Though reported as an acquired abnormality in myeloid neoplasms, it has rarely been described in B-lymphoblastic leukemia which primarily is a disease of the young. We describe here in three cases of pediatric B-lymphoblastic leukemia with LOY. Conventional cytogenetic studies and fluorescence in situ hybridization studies using centromeric probes for chromosome X and Y on peripheral blood samples ruled out constitutional LOY in all the three cases favoring it to be a neoplastic phenomenon.