Bone marrow hematogones (B-lymphocyte precursors) may cause problems in diagnosis because of their morphologic and immunophenotypic similarities to neoplastic lymphoblasts. The purposes of this prospective, multiparametric flow cytometry study were to quantify hematogones across age groups and a spectrum of clinical conditions, to identify factors that affect the relative quantity of hematogones, and to compare their immunophenotype with that of neoplastic lymphoblasts. A total of 662 consecutive marrow specimens were analyzed for hematogones using one of two 4-color antibody combinations; hematogones were identified in 528 (79.8%). There was a significant decline in hematogones with increasing age (P < .001), but a broad range was found at all ages and many adults had a relatively high number. Specimens processed by density gradient had a higher mean percent hematogones than those processed by erythrocyte lysis (P < .001). There was a direct decline in hematogones with increasing marrow involvement with neoplastic cells. A total of 8% of the 662 specimens contained 5% or more hematogones: 24.6% of specimens from patients aged less than 16 years and 6.3% from those 16 and older (P < .000 01). Increased hematogones were observed most often in patients with lymphoma, marrow regenerative states, immune cytopenias, and acquired immunodoficiency syndrome. Hematogones always exhibited a typical complex spectrum of antigen expression that defines the normal antigenic evolution of B-cell precursors and lacked aberrant expression. In contrast, lymphoblasts in 49 cases of precursor B.ALL showed maturation arrest and exhibited I 1 to 11 immunophenotypic aberrancies. Four-color flow cytometry with optimal combinations of antibodies consistently distinguishes between hematogones and neoplastic lymphoblasts.