Mutations in the tyrosine kinase receptor FLT3 are the most common molecular abnormality in acute myeloid leukemia (AML) being detected in about 30% of AML cases. According to the protein domain altered FLT3 mutations may be classified as juxtamembrane or activation loop. The former are caused by internal tandem duplications (ITD) in exons 14 and 15 and is detected in 20–27% of AML patients. Mutations in the activation loop are mainly due to point mutations in exon 20 and is present in 5–7% of AML patients. Most of these mutations lead to changes in the aspartate in position 835 (D835), which have been detected in about 7% of AML cases. Both types of mutations cause the constitutive activation of FLT3 and are associated with bad prognosis. AML characteristics in Latin America are different from those in Europe and US. Namely, there is a higher frequency of acute promyelocytic leukemia (APL) and the clinical outcome of adult patients with other subtypes of AML treated with standard protocols is poorer. The worse prognosis seems to be related to the biology of the disease, rather than socio-economic features, based on studies of other hematological malignancies. In order to test if a higher frequency or different FLT3 mutations might explain these observations, we performed a screening for mutations in FLT3 using PCR and single strand conformation polymorphism (SSCP) techniques to evaluate exons 12 to 20, which encode for the intracytoplasmatic domains of the protein. Ninety-nine consecutive patients with AML (90 adults and 9 children) and 55 blood donors (controls) were analyzed. Two synonyms mutations that have not been previously described were detected: one in exon 12 (T526T) and the other in exon 17 (G697G). ITD mutations were detected in 23 (23.2%) patients with AML, therefore within the expected frequency based on the studies in developed countries. On the other hand, D835 mutations were absent, and except for a mutation detected in one patient causing the deletion of the aminoacid in the position 836 no other abnormality was detected in exon 20. No mutations were detected in exons 13, 16, 18 and 19. In adults, mutations were more frequent in acute promyelocitic leukemia and in women. There were no associations between FLT3 mutations and CBC values. Interestingly, FLT3 mutations were less frequently detected in patients whose leukemic cells expressed the CD56 marker. As described for other countries, overall survival was worse in patients with FLT3 mutations. In conclusion, this study demonstrates that the frequency of ITD mutations in FLT3 gene in Brazilian patients with AML was similar to the frequency described in the European and North American populations, whereas activation loop mutations were rarely detected, especially those in D835. Thus, our results suggest that Brazilian patients with AML have a distinct profile of genetic abnormalities. In addition, this is the first study to demonstrate a negative association between FLT3 mutations and the expression of CD56 by leukemic blasts. This is a relevant observation, since CD56 expression per se has been identified as prognostic factor by other investigators. Finally, our study demonstrates that the SSCP method may be useful for screening mutations of FLT3 gene.