Eighty-two unselected cases of therapy-related myelodysplasia (t-MDS) or acute myeloid leukemia (t-AML) were investigated for internal tandem duplications of the FLT3 gene (FLT3/ITD), for internal tandem duplications of the MLL gene (MLL/ITD) and for mutations of the WT1 gene. FLT3/ITD were observed in three patients, another two patients presented MLL/ITD whereas mutations of the WT1 gene were not observed. All FLT3/ITD included the tyrosine-rich stretch between codons 589 and 599, and both MLL/ITD presented break points within Alu-repeats, as previously observed in de novo AML. The ITD were not related to any specific type of previous therapy, but three out of the five cases were observed among only six patients with overt t-AML and a normal karyotype (P = 0.0043). Interestingly, one of the patients with FLT3/ITD presented overt t-AML of subtype M1 with a normal karyotype after treatment with an alkylating agent. Complete remission was observed following treatment with daunorubicin and cytosine arabinoside, but after 37 months the patient relapsed with t-AML of subtype M3 with a t(15;17) and the same FLT3/ITD was still present. Thus FLT3/ITD may in this case represent a primary event in leukemogenesis, whereas the t(15;17) may represent a secondary event most likely induced by subsequent therapy. In conclusion, FLT3/ITD and MLL/ITD are mainly observed in uncharacteristic cases of t-AML with a normal karyotype and unrelated to previous therapy for which reason they could represent sporadic cases of de novoAML.