Two hundred and nine children presenting consecutively with acute lymphoblastic leukaemia to a regional paediatric oncology unit were investigated to determine the prognostic significance of various factors at diagnosis. There was a strong positive correlation between the pretreatment haemoglobin level and the percentage of bone marrow blast cells in S phase of the cell cycle as assessed by flow cytometry. Patients with T- and B-cell leukaemia had significantly higher haemoglobin levels than non-B non-T patients. In patients with total white cell counts less than 20 X 10(9)/l, aged less than 13 years, and no mediastinal mass, there was no association of haemoglobin with length of first remission. However, among those with white blood counts greater than 20 +/- 10(9)/l there was a strong positive trend towards shorter remission with higher haemoglobin levels. Children with high white blood counts at diagnosis and low haemoglobin levels may have a better prognosis than predicted by the white blood count alone.