We used high-sensitivity flow cytometry to assess 93 bone marrow aspirates for involvement by systemic mastocytosis. Aberrant CD2/CD25 expression by CD117-gated mast cells was seen in 34 samples (37%), with the majority of mast cells expressing both markers (n = 23; 68%). In 24 cases, a discrete population of mast cells within the CD117-bright gate correlated with a positive morphologic finding in the biopsy, even in the absence of an aberrant immunophenotype. A discrete CD117-bright population, when considered a positive criterion, increases analytic sensitivity from 77% to 95%, exceeding the sensitivity of morphologic analysis (69% for aspirate and 85% for biopsy). We conclude that flow cytometry is a sensitive and specific test for the presence of systemic mastocytosis, particularly when the presence of a discrete CD117-positive mast cell population is regarded as a diagnostic criterion.