OBJECTIVE To compare the results of the monocyte count provided by autoanalysers with those attained with flow cytometry. METHODS Fifty-six blood samples (EDTA-K3) from the emergency laboratory were studied. The automatic percentage of monocytes attained by four autoanalysers, STKS (Coulter-IZASA), Technicon H*2 (Bayer Diagnostics), Cell-Dyn-3000 (Abbott Diagnostics) and NE-8000 (TOA-Sysmex) was confronted with that provided by the flow cytometry analysis performed with a FACScan (Becton-Dickinson) using monoclonal antibodies of myeloid-monocytic lineage (CD14, CD16). The morphologic observation of the blood smear in accordance with the H20-A protocol of the National Committee for Clinical Laboratory Standards (NCCLS) was used as a reference method. Descriptive statistics plus linear regression analysis, along with the Student's t test for paired data were used for the statistical evaluation of the results. RESULTS Significant correlation was found between the systems under study and the flow cytometry method: Technicon H*2 (y = 1.79 +/- 0.61x; r = 0.91, p < 0.0001), Coulter STKS (y = 0.61 +/- 0.93X; r = 0.89, p < 0.0001), Cell-Dyn 3000 (y = 0.88 + 0.64x; r = 0.78, p < 0.0001) and Sysmex NE-8000 (y = 4.60 +/- 0.33x; r = 0.52, p < 0.0001). When comparing the percentage of monocytes by means of the Student's t test it was found that Coulter STKS provided the closest results with regard to flow cytometry (13.78 +/- 9.27 vs 14.19 +/- 8.88, p = 0.49), while the closest findings with respect to the reference method were given by Technicon H*2 (9.29 +/- 4.78 vs 9.71 +/- 4.77, p = 0.30). CONCLUSIONS From this analysis, it was concluded that flow cytometry could be an alternative method to conventional optic observation in the evaluation of the differential count of leucocytes, including monocytes.