A group of 15 patients with acute lymphoblastic leukaemia was studied prospectively to analyse the impact of the pretreatment DNA labelling indices of the blast cells. Patients with labelling indices less than 5% did significantly worse than those with labelling indices greater than 5%: the 60-week disease-free survival of patients with acute lymphoblastic leukaemia and labelling indices greater than 5% was 80%, whereas that for patients with labelling indices less than 5% was 24%, P less than 0.05. In addition, three patients with acute leukaemia and DNA labelling indices of 0%, so-called G-0 acute leukaemias, are described: all of them had refractory forms of acute lymphoblastic leukaemia with a very poor outcome to treatment.