Recently, in-frame internal tandem duplications have been reported within the regions coding for the juxtamembrane through the first tyrosine kinase domain of the Flt3 gene. These duplications have been reported to lead to autophosphorylation of the receptor. In this study we investigated the effect of such mutations in the Flt3 gene on the in vitro proliferation of human acute myeloid leukemia cells. The mutations were detected in 10 out of 59 AML bone marrow samples analyzed and were not restricted to a specific FAB class or cytogenetic aberration. PCR analysis of those samples showed all mutations to be present in exon 11 of the gene. Whilst samples without a mutation of the Flt3 gene showed an increased cell production in response to either IL-3 and G-CSF or IL-6, SCF, TPO and Flt3L in long-term stroma supported cultures, mutant samples failed to do so. As we could not find a relationship between the absence of a response and either FAB class or cytogenetic aberrations, we interpret these results as an indication that the internal tandem duplications in the Flt3 gene are the prime cause of this unresponsiveness. Although our study does not explain the mechanism by which these mutations cause this unresponsiveness it does suggest that AML cells need a wild-type Flt3 for optimal in vitro proliferation.