OBJECTIVE: To carry out morphometric analyses of leukemic blast nuclei in patients with acute leukemias and determine whether it is possible to classify respondents into groups based on these karyometric values and their relationship with the FAB morphological subtype. STUDY DESIGN: Bone marrow smears were stained by May-Grünwald Giemsa, and digital image analysis was performed by software ImageJ. For each patient 100 nuclei were examined and parameters of area, perimeter, Ferret's diameter, optical density, and integrated optical density (IOD) were measured. Molecular analysis of FLT3 ITD mutation by polymerase chain reaction were done from native bone marrow smears. RESULTS: Nuclear size and IOD were significantly larger in M4 than in M1 subtype. Differences in optical density were not statistically significant. The highest value of circularity was found in M0 subtype, and lowest values of circularity were found in patients with monoblastic morphology (FAB M5), but these differences were not statistically significant. No statistically significant difference was found in terms of the frequency of FLT3 mutations in different FAB subtypes. Nuclear size was significantly larger in FLT3 ITD mutation-positive patients. CONCLUSION: Karyometric analysis of blasts can be considered as an auxiliary tool alongside standard diagnostic techniques. Lower values of circularity were found in patients with monoblastic morphology (FAB M5). Higher values of nuclear size (perimeter and Ferret's diameter) were found in patients with monocytic morphology (FAB M4/M5). Frequency of FLT3 ITD mutation is not specific to the individual FAB subtypes. Patients with FLT3 ITD mutation have blasts with larger nuclei. © Science Printers and Publishers, Inc.