Genomic DNA from 97 cases of adult de novo acute myeloid leukaemia (AML) was screened using polymerase chain reaction (PCR) and conformation-sensitive gel electrophoresis (CSGE) for FLT3 exon 20 mutations. Initial sequencing of four cases, representing the spectrum of CSGE abnormalities, revealed changes affecting codon Asp835 in three cases and also an intron 20 A to G change. In order to identify all possible Asp835 alterations, as well as the frequency of the intronic change nucleotide 2541 + 57 A-->G, the patient PCR products were digested with EcoRV and NlaIII respectively. Seven cases (7.2%) possessed a mutation affecting Asp835; these were identified, following DNA sequencing, as Asp835Tyr (n = 5), Asp835His (n = 1) and Asp835del (n = 1). Alterations affecting Asp835 were not found in 80 normal control DNA samples. In contrast, the nucleotide 2541 + 57 A-->G change was shown to be a polymorphism, with an allelic frequency of 0.24 for the G and 0.76 for the A allele. This study reports, for the first time, point mutations in the human FLT3 gene that, because of their homology with other class III receptor tyrosine kinase mutations, probably result in constitutive activation of the receptor.