The cytogenetic characteristics of 37 patients diagnosed of acute lymphoblastic leukaemia are presented. The studies were performed by cytofluorometry (CFM) after DNA staining with propidium iodide (34 cases) and/or chromosome identification with trypsin G bands (13 patients). Hyperdiploid DNA index was present in 15% of the patients, whereas none of them had hypodiploid DNA index. Abnormal karyotype was found in 69% of the evaluated cases. Good correlation was observed between the ploidy attained by CFM and by karyotyping cells. The highest percentage of aneuploidy corresponded to the L2 morphological subtype (55%), followed by L1 (36%). Structural alterations were the commonest in L2 variant, while numerical ones were commonest in the L1 variant. The 4 L1 patients with aneuploidy had the common immunophenotype, whereas the 6 aneuploidic patients of the L2 variant had common, early pre-B and undifferentiated immunophenotype. The actuarial survival of patients with diploid DNA index was 48.5% (IC 95%, 25-73%), whereas pseudodiploid patients have relapsed and died before 16 months from diagnosis (p less than 0.005). None of the patients with hyperdiploid DNA index and lacking structural alterations has relapsed. Patients with structural abnormalities have the poorest prognosis, while patients with hyperdiploid DNA index showed several favourable risk factors and are in the first complete remission.