We have analysed the prognostic influence of cytogenetic findings at diagnosis in a group of 502 children with acute lymphoblastic leukaemia (ALL), treated on MRC UKALL X, in whom clonal cytogenetic abnormalities were detected at diagnosis. Despite the overall improvement in outcome in children treated on this protocol compared with previous trials, some cytogenetically-defined groups were still associated with a poor outcome and ploidy retained some prognostic significance. Patients with high hyperdiploid ALL (39% of those with clonal abnormalities) had a favourable outcome with event free survival of 71% at 5 years. Those with near haploidy (1%), hypodiploidy (9%) and low hyperdiploidy (16.5%) had a relatively poor prognosis with event-free survival at 5 years of 17%, 42% and 49% respectively. Only two of 12 children with Ph-positive leukaemia are alive in remission and abnormalities of chromosome 11q23 were also associated with a high risk of treatment failure. In contrast, the t(1;19) was associated with improved event-free survival of 87.5% at 5 years. A number of other non-random abnormalities were identified with no clear prognostic significance. We conclude that identification of certain genetic changes remains important in the management of acute lymphoblastic leukaemia, although whether molecular diagnosis of clinically relevant abnormalities can now supplant cytogenetics in the clinical trials context remains to be determined.