Summary. A chromosomal classification of 93 children with acute lymphoblastic leukaemia (ALL) is presented. The chromosomal categories were normal, hyperdiploid, pseudodiploid and hypodiploid; the chromosomally abnormal cases were classified according to the presence of an abnormal clone. The longest follow-up was 9 years. Infants and older children were over-represented in the pseudodiploid category: this association was statistically significant. Patients in the hyperdiploid and hypodiploid categoreis had the longest first remissions and overall first remissions and overall survival and those in the pseudodiploid category the shortest. These effects were statistically siginigicant even when the effect of age and leucocyte count were taken into account. Thus chromosomal findings at diagnosis in ALL can be used as an independent prognostic factor.