Current risk-adjusted intensive therapies for childhood acute lymphoblastic leukaemia (ALL) are expected to result in an event-free survival of greater than 75%. In sharp contrast, relapsed paediatric ALL is a difficult disease to treat. In this study, 25 paediatric patients with ALL were analysed at diagnosis and relapse for their p16 (exon 2) status using the most accurate method of detection, real-time polymerase chain reaction (PCR). The median time to relapse for the group was 27 months. At diagnosis, the incidence of p16 homozygous and hemizygous deletion in this group was 32% and 20% respectively. The incidence of homozygous p16 deletion at relapse was 64%. A large number of patients, eight of 16 (50%), developed p16 homozygous deletion at relapse. Of those eight patients, four were hemizygous and four were germline at diagnosis. At diagnosis, those patients with a homozygous or hemizygous p16 deletion relapsed sooner than those germline for p16. We have shown that p16 alterations are frequently present in relapsed lymphoblastic leukaemia in children.