In this study we investigated the presence of structural lesions in the ALL-1, p53 and p16 (cyclin-dependent kinase 4 inhibitor) genes in leukaemic cells obtained from 22 patients with infant acute leukaemia (aged < 18 months). Of these, 18 cases were classified as acute lymphoblastic leukaemia (ALL) and four as acute myeloid leukaemia (AML). Tumour DNAs were analysed by a combination of Southern blot. polymerase chain reaction (PCR), single-strand conformation polymorphism (SSCP), and direct sequence analyses. The results showed ALL-1 gene rearrangements in 15/22 (68%) cases, p53 gene mutations in 5/22 (26%), and a homozygous deletion of p16 in a single T-ALL case. p53 and p16 alterations were all found in the group of patients with ALL-1 gene rearrangements. p53 mutations were more often associated with a myeloid phenotype (3/5). In summary, multiple molecular alterations were found in 6/15 (40%) infant acute leukaemias with ALL-1 rearrangements. As to the clinical course, patients with additional lesions had similar clinical outcome with respect to patients with ALL-1 gene rearrangement as the sole genetic aberration. This may support the hypothesis that ALL-1 alterations are genetic events per se sufficient to confer a fully malignant phenotype to the leukaemic clone.