Hematopoietic stem cells (HSCs) give rise to mast cells (MCs), and a relative increase in bone marrow (BM) MC is common in various BM failure (BMF) conditions. Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired clonal HSC disorder, frequently associated with BMF, characterized by decreased expression of glycosylphosphatidylinositol membrane anchor for complement regulatory proteins. Eculizumab, a monoclonal antibody that blocks complement factor 5, successfully controls PNH symptoms. In this study, we, for the first time, demonstrated PNH-type culture-grown BM MC (c-BMMC) using flow cytometry in two BMF patients and monitored population size during eculizumab therapy. Further research may unravel the properties of PNH-type c-BMMC.