Chromosome studies, using bone marrow samples of 26 pretreated children (below 15 years of age) with Acute Lymphoblastic Leukemia were carried out to explore the potentialities of applying chromosomal findings as a prognostic indicator in these patients. Abnormal karyotype was identified in 15 patients (57.6 per cent). The chromosomes frequently involved in non-random numerical abnormalities were Nos. 8, 18 and 21. Structural chromosome changes observed consisted of deletion 6q- and translocation t (4;11). After karyotype analysis, patients were grouped into subsets on the basis of the karyotype pattern observed. They were followed up to evaluate their prognosis and survival period. Patients showing hyperdiploid clone with greater than 51 chromosomes had the best prognosis. Patients with normal karyotype and patients with deletion of the long arm of chromosome 6 showed intermediate prognosis whereas patients showing t (4;11), trisomy 8, trisomy 18, trisomy 21, and hypodiploid karyotype were associated with worst prognosis. Thus, karyotype analysis before treatment helps to classify ALL patients as poor, intermediate and good prognosis groups and on this basis therapy can be designed accordingly.