The purpose of the study was to examine the influence of age on outcome in a large cohort of children and adults with lymphoblastic leukaemia who were treated on two similar trials. Factors influencing outcome were examined in 2204 patients aged over 1 year treated between 1985 and 1992 on the parallel Medical Research Council Trials UKALL X and XA, for children and adults, respectively. There was a progressive worsening in survival with increasing age from 85% (95% CI 83-87) at 5 years for children aged 1-9 to 24% (CI 17-31) for patients over 40. Induction failures, deaths in remission and bone marrow relapses increased significantly with age. Analysis of clinical and biological features showed dominance of early B-ALL in childhood and increasing incidence of the Ph' chromosome with age. Over 80% of eligible children, but a much lower proportion of adults especially those over 40, was entered. Compliance was stricter in the paediatric trial but most deviations in adults involved giving more treatment. Analysis of results in a proportional hazards model confirmed the overwhelming independent influence of age; with all other factors equal a 10 year old had half the risk of treatment failure of a 20 year old and a 44 year old double the risk. Selective entry to therapeutic trials and increased treatment-related toxicity are features of adult ALL but age itself remains a dominant prognostic factor. While improved supportive care and refinements of conventional therapy may have some effect on prognosis, new understandings and treatment approaches to adult ALL are needed.