Flow cytometric analysis is important in the diagnosis, classification, and follow-up of non-Hodgkin lymphoma. It is assumed that the lymphoma phenotype for each patient remains unchanged over time and is consistent from one specimen to another. To determine the variability in expression of lymphoid antigens, we reviewed 211 flow cytometry specimens of malignant lymphoma from 81 patients. Some antigens showed a stable pattern of expression such as CD5, CD10, CD19, CD20, and HLA-DR. In contrast, CD21, CD22, CD23, and CD25 showed more variability from one specimen to another. We believe several factors affect the stability of antigen expression. True differences in expression most probably are related to the biology and function of the different antigens. For instance, CD19 and CD20 are essential in cell maturation and function and, therefore, are present on the majority of cells. In contrast, CD22 has a role during B-cell activation and, therefore, is more variable. Lack of standardization inflow cytometry procedures also is responsible for some variability. Instrument settings for adequate compensation and the criteria used to determine when an antigen is reported as positive are important considerations when evaluating flow cytometry histograms.