Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired disorder of the pluripotent stem cell resulting from the somatic mutation of the X-linked PIG-A gene, involved in the synthesis of the glycosylphosphatidyl-inositol anchor of membrane proteins such as CD55, CD59 and CD14. In the past decade, flow cytometry has become a valuable diagnostic tool in the detection of deficient expression of the GPI-anchored proteins. We report the diagnosis of PNH in four patients confirmed by flow cytometry. Red blood cells, granulocytes and monocytes were classified as PNH types I, II and III according to the mean fluorescence intensities (MFI) of membrane proteins. MFI rate is a numerical data reflecting the severity of decreased antigen expression, and it is obtained by dividing the MFI of the type II or type III cells by the MFI of the respective cells obtained for a normal sample. We found that the investigation of granulocytes and monocytes was more informative than red blood cells when percent negativity was evaluated. In addition, the lowest MFI rate (mean 0.011) was obtained for CD14 on monocytes while CD59 and CD55 gave higher values on all three investigated cell types (0.021-0.34). Thus, CD14 on monocytes seems to be the most reliable marker for establishing the PNH clone size and the severity of antigen deficiency.