Summary. To determine the incidence of homozygous deletions of the newly identified tumour suppressor gene, CDK4I, molecular genomic DNA analyses by PCR technique were performed on primary neoplastic cells from 22 childhood acute leukaemias obtained at presentation. The blast cells derived in all the analysed cases from bone marrow. We found that none of acute myeloblastic leukaemias (four cases) showed the CDK4I alteration, whereas 6/13 (46%) common acute lymphoblastic leukaemias (ALLs) displayed homozygous deletions. Moreover, and even more important, all the blasts purified from ALLs derived from early lymphoid precursors (three early‐T ALLs and two pre‐B ALLs) showed the absence of CDK4I gene. When the entire coding sequence of the CDK4I gene from samples without homozygous deletions was analysed by the single‐strand conformational polymorphism method, no point mutations were identified. These results demonstrate that CDK4I gene deletions are very frequent and probably early events in childhood acute leukaemias of lymphoid origin and especially in early‐T and pre‐B ALLs. Moreover, the molecular mechanism of the loss of function of the gene is correlated, at least in childhood ALLs, almost exclusively to deletions and not to point mutations.