Children who have completed treatment for acute lymphoblastic leukaemia (ALL) are commonly followed up for the first 5 years with regular full blood counts (FBCs) to monitor for relapse of disease. There is little evidence to suggest that this practice improves the detection rate of unexpected relapse. Surveillance FBCs, performed on 43 children with relapsed ALL between 1990 and 1999, were analysed. Of the 42 relapses in children off therapy, only two were detected by an abnormal FBC. Routine FBCs in asymptomatic children off therapy lacks specificity in detecting unexpected relapses and maybe safely discontinued.