Abstract The KIT D816V mutation is detected in the vast majority of adult cases of systemic mastocytosis (SM). The mutation is also frequently detected in core-binding factor acute myeloid leukemia (CBF-AML) defined by the presence of t(8;21)(q22;q22); RUNX1-RUNX1T1 or inv(16)(p13.1;q22)/t(16;16)(p13.1;q22); CBFB-MYH11 chromosomal rearrangements, but whether the mutation is indicative of associated SM is unclear. In the present study, CBF-AML patients were therefore analysed for the KIT D816V mutation and mutation positive cases subsequently analysed for the presence of SM. The KIT D816V mutation was detected in 8 of 20 cases of CBF-AML with the frequency in t(8;21)(q22;q22) and inv(16)(p13.1;q22) positive cases being 31% and 57%, respectively. The fraction of KIT D816V mutation positive cells was highly variable among the 8 mutation positive patients with levels ranging from 0.04 - 98% in a pre-treatment blood sample. Five of the 8 cases carried the mutation in a cell fraction below one-tenth of the blast cell fraction, thus suggesting that KIT mutation is often a late event in leukemogenesis. None of the 8 KIT D816V mutation positive cases fulfilled the WHO diagnostic criteria of SM. The presence of the KIT D816V mutation in the CBF-AML subgroup can therefore not be considered indicative of associated SM.