Isolated extramedullary manifestations (IEM) of acute myeloid leukemia (AML) are recurrent events, especially following allogeneic hematopoietic cell transplantation (alloHCT). To date, measurable residual disease (MRD) assessment for this difficult-to-treat patient cohort has not been established. In this study, we evaluated highly sensitive next-generation sequencing (NGS) of IEM-AML tumor and compared it with cell-free DNA (cfDNA) from plasma, as well as highly sensitive NGS analysis of bone marrow mononuclear cells (BMMC) and peripheral blood mononuclear cells (PBMC), in a cohort of 15 IEM-AML patients with 19 IEM-AML episodes. cfDNA demonstrated a superior representation of IEM-AML tumor mutations compared to BMMC or PBMC, with a median variant allele frequency (VAF) of 0.8% and a mutation detection rate of 62% (37 of 60 mutations), compared to a median VAF of 0.05% and detection rate of 27%, respectively (16 of 60 mutations,